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style="margin-top:4px;margin-bottom:4px"><div class="css-fhn3dm MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-w8t2sr"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-1guu2k2" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ChevronRightIcon"><path d="M10 6 8.59 7.41 13.17 12l-4.58 4.59L10 18l6-6z"/></svg><div class="css-szjynw"><div class="MuiBox-root css-1uoa4hf"><div class="MuiTypography-root MuiTypography-body1 css-1g11p7i">TiDB 工具</div></div></div></div></div></div></li><li class="MuiTreeItem-root css-1p2jk91" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-0-13" tabindex="-1" style="margin-top:4px;margin-bottom:4px"><div class="css-fhn3dm MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-w8t2sr"><svg class="MuiSvgIcon-root 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class="MuiBox-root css-1uoa4hf"><div class="MuiTypography-root MuiTypography-body1 css-1g11p7i">常见问题解答 (FAQ)</div></div></div></div></div></div></li><li class="MuiTreeItem-root css-1p2jk91" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-0-15" tabindex="-1" style="margin-top:4px;margin-bottom:4px"><div class="css-fhn3dm MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-w8t2sr"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-1guu2k2" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ChevronRightIcon"><path d="M10 6 8.59 7.41 13.17 12l-4.58 4.59L10 18l6-6z"/></svg><div class="css-szjynw"><div class="MuiBox-root css-1uoa4hf"><div class="MuiTypography-root MuiTypography-body1 css-1g11p7i">版本发布历史</div></div></div></div></div></div></li><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/glossary/"><li class="MuiTreeItem-root css-1p2jk91" role="treeitem" id="left-nav-treeview-0-0-16" tabindex="-1" style="margin-top:4px;margin-bottom:4px"><div class="css-fhn3dm MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-w8t2sr"><div class="MuiBox-root css-f81823"/><div class="css-szjynw"><div class="MuiBox-root css-1uoa4hf"><div class="MuiTypography-root MuiTypography-body1 css-1g11p7i">术语表</div></div></div></div></div></div></li></a></ul></div></aside><main class="MuiBox-root css-sb2sd0"><div class="MuiBox-root css-w0vdto"><div class="css-1q5cbag"><div class="MuiBox-root css-was4pq"><div class="MuiContainer-root MuiContainer-maxWidthLg MuiContainer-disableGutters doc-content css-5csqms"><div class="markdown-body MuiBox-root css-0"><h1 id="optimizer-hints" style="position:relative">Optimizer Hints<a href="#optimizer-hints" aria-label="optimizer hints permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h1><p>TiDB 支持 Optimizer Hints 语法，它基于 MySQL 5.7 中介绍的类似 comment 的语法，例如 <code>/*+ HINT_NAME(t1, t2) */</code>。当 TiDB 优化器选择的不是最优查询计划时，建议使用 Optimizer Hints。</p><p>如果遇到 Hint 无法生效的情况，请参考<a href="#%E5%B8%B8%E8%A7%81-hint-%E4%B8%8D%E7%94%9F%E6%95%88%E9%97%AE%E9%A2%98%E6%8E%92%E6%9F%A5">常见 Hint 不生效问题排查</a>。</p><h2 id="语法" style="position:relative">语法<a href="#%E8%AF%AD%E6%B3%95" aria-label="语法 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h2><p>Optimizer Hints 不区分大小写，通过 <code>/*+ ... */</code> 注释的形式跟在 <code>SELECT</code>、<code>INSERT</code>、<code>UPDATE</code> 或 <code>DELETE</code> 关键字的后面。</p><p>多个不同的 Hint 之间需用逗号隔开，例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ USE_INDEX(t1, idx1), HASH_AGG(), HASH_JOIN(t1) */</span> <span class="hljs-built_in">count</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t t1, t t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t2.b; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>可以通过 <a href="/zh/tidb/stable/sql-statement-explain/"><code>Explain</code></a>/<a href="/zh/tidb/stable/sql-statement-explain-analyze/"><code>Explain Analyze</code></a> 语句的输出，来查看 Optimizer Hints 对查询执行计划的影响。</p><p>如果 Optimizer Hints 包含语法错误或不完整，查询语句不会报错，而是按照没有 Optimizer Hints 的情况执行。如果 Hint 不适用于当前语句，TiDB 会返回 Warning，用户可以在查询结束后通过 <code>Show Warnings</code> 命令查看具体信息。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>如果注释不是跟在指定的关键字后，会被当作是普通的 MySQL comment，注释不会生效，且不会上报 warning。</p></div></div></div><p>TiDB 目前支持的 Optimizer Hints 根据生效范围的不同可以划分为两类：第一类是在查询块范围生效的 Hint，例如 <a href="#hash_agg"><code>/*+ HASH_AGG() */</code></a>；第二类是在整个查询范围生效的 Hint，例如 <a href="#memory_quotan"><code>/*+ MEMORY_QUOTA(1024 MB)*/</code></a>。</p><p>每条语句中每一个查询和子查询都对应着一个不同的查询块，每个查询块有自己对应的名字。以下面这条语句为例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t) t1, (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t) t2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>该查询语句有 3 个查询块，最外面一层 <code>SELECT</code> 所在的查询块的名字为 <code>sel_1</code>，两个 <code>SELECT</code> 子查询的名字依次为 <code>sel_2</code> 和 <code>sel_3</code>。其中数字序号根据 <code>SELECT</code> 出现的位置从左到右计数。如果分别用 <code>DELETE</code> 和 <code>UPDATE</code> 查询替代第一个 <code>SELECT</code> 查询，则对应的查询块名字分别为 <code>del_1</code> 和 <code>upd_1</code>。</p><h2 id="查询块范围生效的-hint" style="position:relative">查询块范围生效的 Hint<a href="#%E6%9F%A5%E8%AF%A2%E5%9D%97%E8%8C%83%E5%9B%B4%E7%94%9F%E6%95%88%E7%9A%84-hint" aria-label="查询块范围生效的 hint permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h2><p>这类 Hint 可以跟在查询语句中<strong>任意</strong> <code>SELECT</code>、<code>UPDATE</code> 或 <code>DELETE</code> 关键字的后面。通过在 Hint 中使用查询块名字可以控制 Hint 的生效范围，以及准确标识查询中的每一个表（有可能表的名字或者别名相同），方便明确 Hint 的参数指向。若不显式地在 Hint 中指定查询块，Hint 默认作用于当前查询块。以如下查询为例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ HASH_JOIN(@sel_1 t1@sel_1, t3) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> (<span class="hljs-keyword">SELECT</span> t1.a, t1.b <span class="hljs-keyword">FROM</span> t t1, t t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t2.a) t1, t t3 <span class="hljs-keyword">WHERE</span> t1.b <span class="hljs-operator">=</span> t3.b; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>该 Hint 在 <code>sel_1</code> 这个查询块中生效，参数分别为 <code>sel_1</code> 中的 <code>t1</code> 表（<code>sel_2</code> 中也有一个 <code>t1</code> 表）和 <code>t3</code> 表。</p><p>如上例所述，在 Hint 中使用查询块名字的方式有两种：第一种是作为 Hint 的第一个参数，与其他参数用空格隔开。除 <code>QB_NAME</code> 外，本节所列的所有 Hint 除自身明确列出的参数外都有一个隐藏的可选参数 <code>@QB_NAME</code>，通过使用这个参数可以指定该 Hint 的生效范围；第二种在 Hint 中使用查询块名字的方式是在参数中的某一个表名后面加 <code>@QB_NAME</code>，用以明确指出该参数是哪个查询块中的表。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>Hint 声明的位置必须在指定生效的查询块之中或之前，不能是在之后的查询块中，否则无法生效。</p></div></div></div><h3 id="qb_name" style="position:relative">QB_NAME<a href="#qb_name" aria-label="qb_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>当查询语句是包含多层嵌套子查询的复杂语句时，识别某个查询块的序号和名字很可能会出错，Hint <code>QB_NAME</code> 可以方便我们使用查询块。<code>QB_NAME</code> 是 Query Block Name 的缩写，用于为某个查询块指定新的名字，同时查询块原本默认的名字依然有效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(QB1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t) t1, (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t) t2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>这条 Hint 将最外层 <code>SELECT</code> 查询块的命名为 <code>QB1</code>，此时 <code>QB1</code> 和默认名称 <code>sel_1</code> 对于这个查询块来说都是有效的。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>上述例子中，如果指定的 <code>QB_NAME</code> 为 <code>sel_2</code>，并且不给原本 <code>sel_2</code> 对应的第二个查询块指定新的 <code>QB_NAME</code>，则第二个查询块的默认名字 <code>sel_2</code> 会失效。</p></div></div></div><h3 id="merge_joint1_name--tl_name-" style="position:relative">MERGE_JOIN(t1_name [, tl_name ...])<a href="#merge_joint1_name--tl_name-" aria-label="merge_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>MERGE_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Sort Merge Join 算法。这个算法通常会占用更少的内存，但执行时间会更久。当数据量太大，或系统内存不足时，建议尝试使用。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MERGE_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>MERGE_JOIN</code> 的别名是 <code>TIDB_SMJ</code>，在 3.0.x 及之前版本仅支持使用该别名；之后的版本同时支持使用这两种名称，但推荐使用 <code>MERGE_JOIN</code>。</p></div></div></div><h3 id="no_merge_joint1_name--tl_name-" style="position:relative">NO_MERGE_JOIN(t1_name [, tl_name ...])<a href="#no_merge_joint1_name--tl_name-" aria-label="no_merge_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_MERGE_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表不要使用 Sort Merge Join 算法。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_MERGE_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="inl_joint1_name--tl_name-" style="position:relative">INL_JOIN(t1_name [, tl_name ...])<a href="#inl_joint1_name--tl_name-" aria-label="inl_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>部分情况下 <code>INL_JOIN</code> Hint 可能无法生效，详情请参阅 <a href="#inl_join-hint-%E4%B8%8D%E7%94%9F%E6%95%88"><code>INL_JOIN</code> Hint 不生效</a>。</p></div></div></div><p><code>INL_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Index Nested Loop Join 算法。这个算法可能会在某些场景更快，消耗更少系统资源，有的场景会更慢，消耗更多系统资源。对于外表经过 WHERE 条件过滤后结果集较小（小于 1 万行）的场景，可以尝试使用。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ INL_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2, t3 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id <span class="hljs-keyword">AND</span> t2.id <span class="hljs-operator">=</span> t3.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>在上面的 SQL 中，<code>INL_JOIN(t1, t2)</code> 会提示优化器对 <code>t1</code> 和 <code>t2</code> 使用 Index Nested Loop Join 算法。注意它并不是指 <code>t1</code> 和 <code>t2</code> 之间使用 Index Nested Loop Join 算法，而是 <code>t1</code> 和 <code>t2</code> 分别与其他表 (<code>t3</code>) 之间使用 Index Nested Loop Join 算法。</p><p><code>INL_JOIN()</code> 中的参数是建立查询计划时内表的候选表，比如 <code>INL_JOIN(t1)</code> 只会考虑使用 <code>t1</code> 作为内表构建查询计划。表如果指定了别名，就只能使用表的别名作为 <code>INL_JOIN()</code> 的参数；如果没有指定别名，则用表的本名作为其参数。比如在 <code>SELECT /*+ INL_JOIN(t1) */ * FROM t t1, t t2 WHERE t1.a = t2.b;</code> 中，<code>INL_JOIN()</code> 的参数只能使用 <code>t</code> 的别名 <code>t1</code> 或 <code>t2</code>，不能用 <code>t</code>。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>INL_JOIN</code> 的别名是 <code>TIDB_INLJ</code>，在 3.0.x 及之前版本仅支持使用该别名；之后的版本同时支持使用这两种名称，但推荐使用 <code>INL_JOIN</code>。</p></div></div></div><h3 id="no_index_joint1_name--tl_name-" style="position:relative">NO_INDEX_JOIN(t1_name [, tl_name ...])<a href="#no_index_joint1_name--tl_name-" aria-label="no_index_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_INDEX_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表不要使用 Index Nested Loop Join 算法。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_INDEX_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="inl_hash_join" style="position:relative">INL_HASH_JOIN<a href="#inl_hash_join" aria-label="inl_hash_join permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>INL_HASH_JOIN(t1_name [, tl_name])</code> 提示优化器使用 Index Nested Loop Hash Join 算法。该算法与 Index Nested Loop Join 使用条件完全一样，两者的区别是 <code>INL_JOIN</code> 会在连接的内表上建哈希表，而 <code>INL_HASH_JOIN</code> 会在连接的外表上建哈希表，后者对于内存的使用是有固定上限的，而前者使用的内存使用取决于内表匹配到的行数。</p><h3 id="no_index_hash_joint1_name--tl_name-" style="position:relative">NO_INDEX_HASH_JOIN(t1_name [, tl_name ...])<a href="#no_index_hash_joint1_name--tl_name-" aria-label="no_index_hash_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_INDEX_HASH_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表不要使用 Index Nested Loop Hash Join 算法。</p><h3 id="inl_merge_join" style="position:relative">INL_MERGE_JOIN<a href="#inl_merge_join" aria-label="inl_merge_join permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>INL_MERGE_JOIN(t1_name [, tl_name])</code> 提示优化器使用 Index Nested Loop Merge Join 算法，该算法与 Index Nested Loop Join 使用条件完全一样。</p><h3 id="no_index_merge_joint1_name--tl_name-" style="position:relative">NO_INDEX_MERGE_JOIN(t1_name [, tl_name ...])<a href="#no_index_merge_joint1_name--tl_name-" aria-label="no_index_merge_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_INDEX_MERGE_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表不要使用 Index Nested Loop Merge Join 算法。</p><h3 id="hash_joint1_name--tl_name-" style="position:relative">HASH_JOIN(t1_name [, tl_name ...])<a href="#hash_joint1_name--tl_name-" aria-label="hash_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>HASH_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Hash Join 算法。这个算法多线程并发执行，执行速度较快，但会消耗较多内存。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ HASH_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>HASH_JOIN</code> 的别名是 <code>TIDB_HJ</code>，在 3.0.x 及之前版本仅支持使用该别名；之后的版本同时支持使用这两种名称，推荐使用 <code>HASH_JOIN</code>。</p></div></div></div><h3 id="no_hash_joint1_name--tl_name-" style="position:relative">NO_HASH_JOIN(t1_name [, tl_name ...])<a href="#no_hash_joint1_name--tl_name-" aria-label="no_hash_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_HASH_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表不要使用 Hash Join 算法。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_HASH_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="hash_join_buildt1_name--tl_name-" style="position:relative">HASH_JOIN_BUILD(t1_name [, tl_name ...])<a href="#hash_join_buildt1_name--tl_name-" aria-label="hash_join_buildt1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>HASH_JOIN_BUILD(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Hash Join 算法，同时将指定表作为 Hash Join 算法的 Build 端，即用指定表来构建哈希表。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ HASH_JOIN_BUILD(t1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="hash_join_probet1_name--tl_name-" style="position:relative">HASH_JOIN_PROBE(t1_name [, tl_name ...])<a href="#hash_join_probet1_name--tl_name-" aria-label="hash_join_probet1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>HASH_JOIN_PROBE(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Hash Join 算法，同时将指定表作为 Hash Join 算法的探测（Probe）端，即用指定表作为探测端来执行 Hash Join 算法。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ HASH_JOIN_PROBE(t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="semi_join_rewrite" style="position:relative">SEMI_JOIN_REWRITE()<a href="#semi_join_rewrite" aria-label="semi_join_rewrite permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>SEMI_JOIN_REWRITE()</code> 提示优化器将查询语句中的半连接 (Semi Join) 改写为普通的内连接。目前该 Hint 只作用于 <code>EXISTS</code> 子查询。</p><p>如果不使用该 Hint 进行改写，Semi Join 在选择 Hash Join 的执行方式时，只能够使用子查询构建哈希表，因此在子查询比外查询结果集大时，执行速度可能会不及预期。Semi Join 在选择 Index Join 的执行方式时，只能够使用外查询作为驱动表，因此在子查询比外查询结果集小时，执行速度可能会不及预期。</p><p>在使用了 <code>SEMI_JOIN_REWRITE()</code> 进行改写后，优化器便可以扩大选择范围，选择更好的执行方式。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 不使用 SEMI_JOIN_REWRITE() 进行改写</span> EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> <span class="hljs-keyword">EXISTS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-number">1</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t.a); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------+---------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------+---------------------------------------------------+</span> <span class="hljs-operator">|</span> MergeJoin_9 <span class="hljs-operator">|</span> <span class="hljs-number">7992.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> semi <span class="hljs-keyword">join</span>, <span class="hljs-keyword">left</span> key:test.t.a, <span class="hljs-keyword">right</span> key:test.t1.a <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexReader_25(Build) <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_24 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_24 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1, index:idx(a) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexReader_23(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_22 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexFullScan_22 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t, index:idx(a) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------+---------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 使用 SEMI_JOIN_REWRITE() 进行改写</span> EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> <span class="hljs-keyword">EXISTS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ SEMI_JOIN_REWRITE() */</span> <span class="hljs-number">1</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t.a); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_16 <span class="hljs-operator">|</span> <span class="hljs-number">1.25</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:IndexReader_15, <span class="hljs-keyword">outer</span> key:test.t1.a, <span class="hljs-keyword">inner</span> key:test.t.a, equal cond:eq(test.t1.a, test.t.a) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─StreamAgg_39(Build) <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.t1.a, funcs:firstrow(test.t1.a)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span>test.t1.a <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexReader_34 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1, index:idx(a) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexReader_15(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">1.25</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:Selection_14 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Selection_14 <span class="hljs-operator">|</span> <span class="hljs-number">1.25</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">not</span>(isnull(test.t.a)) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexRangeScan_13 <span class="hljs-operator">|</span> <span class="hljs-number">1.25</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t, index:idx(a) <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [eq(test.t.a, test.t1.a)], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+------------------------+---------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>在上述例子中可以看到，在使用了 Hint 之后，TiDB 可以选择由表 <code>t1</code> 作为驱动表的 IndexJoin 的执行方式。</p><h3 id="shuffle_joint1_name--tl_name-" style="position:relative">SHUFFLE_JOIN(t1_name [, tl_name ...])<a href="#shuffle_joint1_name--tl_name-" aria-label="shuffle_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>SHUFFLE_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Shuffle Join 算法，该 Hint 只在 MPP 模式下生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ SHUFFLE_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li>使用该 Hint 前，需要保证当前 TiDB 集群能够支持在查询中使用 TiFlash MPP 模式，具体细节见文档<a href="/zh/tidb/stable/use-tiflash-mpp-mode/">使用 TiFlash MPP 模式</a>。</li><li>该 Hint 能与 <a href="#hash_join_buildt1_name--tl_name-"><code>HASH_JOIN_BUILD</code> Hint</a> 和 <a href="#hash_join_probet1_name--tl_name-"><code>HASH_JOIN_PROBE</code> Hint</a> 组合使用，达到控制 Shuffle Join 算法的 Build 端和 Probe 端的作用。</li></ul></div></div></div><h3 id="broadcast_joint1_name--tl_name-" style="position:relative">BROADCAST_JOIN(t1_name [, tl_name ...])<a href="#broadcast_joint1_name--tl_name-" aria-label="broadcast_joint1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>BROADCAST_JOIN(t1_name [, tl_name ...])</code> 提示优化器对指定表使用 Broadcast Join 算法，该 Hint 只在 MPP 模式下生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ BROADCAST_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li>使用该 Hint 前，需要保证当前 TiDB 集群能够支持在查询中使用 TiFlash MPP 模式，具体细节见文档<a href="/zh/tidb/stable/use-tiflash-mpp-mode/">使用 TiFlash MPP 模式</a>。</li><li>该 Hint 能与 <a href="#hash_join_buildt1_name--tl_name-"><code>HASH_JOIN_BUILD</code> Hint</a> 和 <a href="#hash_join_probet1_name--tl_name-"><code>HASH_JOIN_PROBE</code> Hint</a> 组合使用，达到控制 Broadcast Join 算法的 Build 端和 Probe 端的作用。</li></ul></div></div></div><h3 id="no_decorrelate" style="position:relative">NO_DECORRELATE()<a href="#no_decorrelate" aria-label="no_decorrelate permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_DECORRELATE()</code> 提示优化器不要尝试解除指定查询块中对应子查询的关联。该 Hint 适用于包含关联列的 <code>EXISTS</code>、<code>IN</code>、<code>ANY</code>、<code>ALL</code>、<code>SOME</code> 和标量子查询，即关联子查询。</p><p>将该 Hint 写在一个查询块中后，对于该子查询和其外部查询块之间的关联列，优化器将不再尝试解除关联，而是始终使用 Apply 算子来执行查询。</p><p>默认情况下，TiDB 会尝试对关联子查询<a href="/zh/tidb/stable/correlated-subquery-optimization/">解除关联</a>，以达到更高的执行效率。但是在<a href="/zh/tidb/stable/correlated-subquery-optimization/#限制">一部分场景</a>下，解除关联反而会降低执行效率。这种情况下，可以使用该 Hint 来人工提示优化器不要进行解除关联操作。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> t1(a <span class="hljs-type">int</span>, b <span class="hljs-type">int</span>); <span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> t2(a <span class="hljs-type">int</span>, b <span class="hljs-type">int</span>, index idx(b)); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 不使用 NO_DECORRELATE()</span> explain <span class="hljs-keyword">select</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">from</span> t1 <span class="hljs-keyword">where</span> t1.a <span class="hljs-operator">&lt;</span> (<span class="hljs-keyword">select</span> <span class="hljs-built_in">sum</span>(t2.a) <span class="hljs-keyword">from</span> t2 <span class="hljs-keyword">where</span> t2.b <span class="hljs-operator">=</span> t1.b); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_11 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, equal:[eq(test.t1.b, test.t2.b)], other cond:lt(<span class="hljs-built_in">cast</span>(test.t1.a, <span class="hljs-type">decimal</span>(<span class="hljs-number">10</span>,<span class="hljs-number">0</span>) <span class="hljs-type">BINARY</span>), <span class="hljs-keyword">Column</span>#<span class="hljs-number">7</span>) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─HashAgg_23(Build) <span class="hljs-operator">|</span> <span class="hljs-number">7992.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.t2.b, funcs:<span class="hljs-built_in">sum</span>(<span class="hljs-keyword">Column</span>#<span class="hljs-number">8</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">7</span>, funcs:firstrow(test.t2.b)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span>test.t2.b <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableReader_24 <span class="hljs-operator">|</span> <span class="hljs-number">7992.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:HashAgg_16 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─HashAgg_16 <span class="hljs-operator">|</span> <span class="hljs-number">7992.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.t2.b, funcs:<span class="hljs-built_in">sum</span>(test.t2.a)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">8</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─Selection_22 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">not</span>(isnull(test.t2.b)) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_21 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_15(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:Selection_14 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Selection_14 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">not</span>(isnull(test.t1.b)) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_13 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>从以上执行计划中可以发现，优化器自动解除了关联。解除关联之后的执行计划不包含 Apply 算子，取而代之的是子查询和外部查询块之间的 Join 运算，而原本的带有关联列的过滤条件 <code>t2.b = t1.b</code> 也变成了一个普通的 join 条件。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 使用 NO_DECORRELATE()</span> explain <span class="hljs-keyword">select</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">from</span> t1 <span class="hljs-keyword">where</span> t1.a <span class="hljs-operator">&lt;</span> (<span class="hljs-keyword">select</span> <span class="hljs-comment">/*+ NO_DECORRELATE() */</span> <span class="hljs-built_in">sum</span>(t2.a) <span class="hljs-keyword">from</span> t2 <span class="hljs-keyword">where</span> t2.b <span class="hljs-operator">=</span> t1.b); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Projection_10 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t1.a, test.t1.b <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Apply_12 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> CARTESIAN <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, other cond:lt(<span class="hljs-built_in">cast</span>(test.t1.a, <span class="hljs-type">decimal</span>(<span class="hljs-number">10</span>,<span class="hljs-number">0</span>) <span class="hljs-type">BINARY</span>), <span class="hljs-keyword">Column</span>#<span class="hljs-number">7</span>) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_14(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_13 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_13 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─MaxOneRow_15(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─StreamAgg_20 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> funcs:<span class="hljs-built_in">sum</span>(<span class="hljs-keyword">Column</span>#<span class="hljs-number">14</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">7</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Projection_45 <span class="hljs-operator">|</span> <span class="hljs-number">100000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-built_in">cast</span>(test.t2.a, <span class="hljs-type">decimal</span>(<span class="hljs-number">10</span>,<span class="hljs-number">0</span>) <span class="hljs-type">BINARY</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">14</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexLookUp_44 <span class="hljs-operator">|</span> <span class="hljs-number">100000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexRangeScan_42(Build) <span class="hljs-operator">|</span> <span class="hljs-number">100000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2, index:idx(b) <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [eq(test.t2.b, test.t1.b)], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRowIDScan_43(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">100000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------------------+-----------+-----------+------------------------+--------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>从以上执行计划中可以发现，优化器没有解除关联。执行计划中包含 Apply 算子，而带有关联列的条件 <code>t2.b = t1.b</code> 仍然是访问 <code>t2</code> 表时的过滤条件。</p><h3 id="hash_agg" style="position:relative">HASH_AGG()<a href="#hash_agg" aria-label="hash_agg permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>HASH_AGG()</code> 提示优化器对指定查询块中所有聚合函数使用 Hash Aggregation 算法。这个算法多线程并发执行，执行速度较快，但会消耗较多内存。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ HASH_AGG() */</span> <span class="hljs-built_in">count</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">GROUP</span> <span class="hljs-keyword">BY</span> t1.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="stream_agg" style="position:relative">STREAM_AGG()<a href="#stream_agg" aria-label="stream_agg permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>STREAM_AGG()</code> 提示优化器对指定查询块中所有聚合函数使用 Stream Aggregation 算法。这个算法通常会占用更少的内存，但执行时间会更久。数据量太大，或系统内存不足时，建议尝试使用。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ STREAM_AGG() */</span> <span class="hljs-built_in">count</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">GROUP</span> <span class="hljs-keyword">BY</span> t1.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="mpp_1phase_agg" style="position:relative">MPP_1PHASE_AGG()<a href="#mpp_1phase_agg" aria-label="mpp_1phase_agg permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>MPP_1PHASE_AGG()</code> 提示优化器对指定查询块中所有聚合函数使用一阶段聚合算法，该 Hint 只在 MPP 模式下生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MPP_1PHASE_AGG() */</span> <span class="hljs-built_in">COUNT</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">GROUP</span> <span class="hljs-keyword">BY</span> t1.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>使用该 Hint 前，需要保证当前 TiDB 集群能够支持在查询中使用 TiFlash MPP 模式，具体细节见文档<a href="/zh/tidb/stable/use-tiflash-mpp-mode/">使用 TiFlash MPP 模式</a>。</p></div></div></div><h3 id="mpp_2phase_agg" style="position:relative">MPP_2PHASE_AGG()<a href="#mpp_2phase_agg" aria-label="mpp_2phase_agg permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>MPP_2PHASE_AGG()</code> 提示优化器对指定查询块中所有聚合函数使用二阶段聚合算法，该 Hint 只在 MPP 模式下生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MPP_2PHASE_AGG() */</span> <span class="hljs-built_in">COUNT</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">GROUP</span> <span class="hljs-keyword">BY</span> t1.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>使用该 Hint 前，需要保证当前 TiDB 集群能够支持在查询中使用 TiFlash MPP 模式，具体细节见文档<a href="/zh/tidb/stable/use-tiflash-mpp-mode/">使用 TiFlash MPP 模式</a>。</p></div></div></div><h3 id="use_indext1_name-idx1_name--idx2_name-" style="position:relative">USE_INDEX(t1_name, idx1_name [, idx2_name ...])<a href="#use_indext1_name-idx1_name--idx2_name-" aria-label="use_indext1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>USE_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器对指定表仅使用给出的索引。</p><p>下面例子的效果等价于 <code>SELECT * FROM t t1 use index(idx1, idx2);</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ USE_INDEX(t1, idx1, idx2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>当该 Hint 中只指定表名，不指定索引名时，表示不考虑使用任何索引，而是选择全表扫。</p></div></div></div><h3 id="force_indext1_name-idx1_name--idx2_name-" style="position:relative">FORCE_INDEX(t1_name, idx1_name [, idx2_name ...])<a href="#force_indext1_name-idx1_name--idx2_name-" aria-label="force_indext1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>FORCE_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器对指定表仅使用给出的索引。</p><p><code>FORCE_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 的使用方法、作用和 <code>USE_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 相同。</p><p>以下四个查询语句的效果相同：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ USE_INDEX(t, idx1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t; <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ FORCE_INDEX(t, idx1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t; <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t use index(idx1); <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t force index(idx1); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="ignore_indext1_name-idx1_name--idx2_name-" style="position:relative">IGNORE_INDEX(t1_name, idx1_name [, idx2_name ...])<a href="#ignore_indext1_name-idx1_name--idx2_name-" aria-label="ignore_indext1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>IGNORE_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器对指定表忽略给出的索引。</p><p>下面例子的效果等价于 <code>SELECT * FROM t t1 ignore index(idx1, idx2);</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ IGNORE_INDEX(t1, idx1, idx2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t t1; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="order_indext1_name-idx1_name--idx2_name-" style="position:relative">ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])<a href="#order_indext1_name-idx1_name--idx2_name-" aria-label="order_indext1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器对指定表仅使用给出的索引，并且按顺序读取指定的索引。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardError MuiAlert-standard css-kdwypz" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ErrorOutlineIcon"><path d="M11 15h2v2h-2zm0-8h2v6h-2zm.99-5C6.47 2 2 6.48 2 12s4.47 10 9.99 10C17.52 22 22 17.52 22 12S17.52 2 11.99 2zM12 20c-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8-3.58 8-8 8z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">警告</div><p>这个 hint 有可能会导致 SQL 语句报错，建议先进行测试。如果测试时发生报错，请移除该 Hint。如果测试时运行正常，则可以继续使用。</p></div></div></div><p>此 hint 通常应用在下面这种场景中：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>, key(a), key(b)); EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ ORDER_INDEX(t, a) */</span> a <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">ORDER</span> <span class="hljs-keyword">BY</span> a LIMIT <span class="hljs-number">10</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+---------+-----------+---------------------+-------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+---------+-----------+---------------------+-------------------------------+</span> <span class="hljs-operator">|</span> Limit_10 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">offset</span>:<span class="hljs-number">0</span>, count:<span class="hljs-number">10</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexReader_14 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:Limit_13 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Limit_13 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">offset</span>:<span class="hljs-number">0</span>, count:<span class="hljs-number">10</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexFullScan_12 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t, index:a(a) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+---------+-----------+---------------------+-------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>优化器对该查询会生成两类计划：<code>Limit + IndexScan(keep order: true)</code> 和 <code>TopN + IndexScan(keep order: false)</code>，当使用了 <code>ORDER_INDEX</code> Hint，优化器会选择前一种按照顺序读取索引的计划。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li><p>如果查询本身并不需要按顺序读取索引，即在不使用 Hint 的前提下，优化器在任何情况下都不会生成按顺序读取索引的计划。此时，如果指定了 <code>ORDER_INDEX</code> Hint，会出现报错 <code>Can't find a proper physical plan for this query</code>，此时应考虑移除对应的 <code>ORDER_INDEX</code> Hint。</p></li><li><p>分区表上的索引无法支持按顺序读取，所以不应该对分区表及其相关的索引使用 <code>ORDER_INDEX</code> Hint。</p></li></ul></div></div></div><h3 id="no_order_indext1_name-idx1_name--idx2_name-" style="position:relative">NO_ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])<a href="#no_order_indext1_name-idx1_name--idx2_name-" aria-label="no_order_indext1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_ORDER_INDEX(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器对指定表仅使用给出的索引，并且不按顺序读取指定的索引。通常应用在下面这种场景中:</p><p>以下示例中查询语句的效果等价于 <code>SELECT * FROM t t1 use index(idx1, idx2);</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>, key(a), key(b)); EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_ORDER_INDEX(t, a) */</span> a <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">ORDER</span> <span class="hljs-keyword">BY</span> a LIMIT <span class="hljs-number">10</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+----------+-----------+---------------------+--------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+----------+-----------+---------------------+--------------------------------+</span> <span class="hljs-operator">|</span> TopN_7 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t.a, <span class="hljs-keyword">offset</span>:<span class="hljs-number">0</span>, count:<span class="hljs-number">10</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexReader_14 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:TopN_13 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TopN_13 <span class="hljs-operator">|</span> <span class="hljs-number">10.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t.a, <span class="hljs-keyword">offset</span>:<span class="hljs-number">0</span>, count:<span class="hljs-number">10</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexFullScan_12 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t, index:a(a) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------------+----------+-----------+---------------------+--------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>和 <code>ORDER_INDEX</code> Hint 的示例相同，优化器对该查询会生成两类计划：<code>Limit + IndexScan(keep order: true)</code> 和 <code>TopN + IndexScan(keep order: false)</code>，当使用了 <code>NO_ORDER_INDEX</code> Hint，优化器会选择后一种不按照顺序读取索引的计划。</p><h3 id="agg_to_cop" style="position:relative">AGG_TO_COP()<a href="#agg_to_cop" aria-label="agg_to_cop permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>AGG_TO_COP()</code> 提示优化器将指定查询块中的聚合函数下推到 coprocessor。如果优化器没有下推某些适合下推的聚合函数，建议尝试使用。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ AGG_TO_COP() */</span> <span class="hljs-built_in">sum</span>(t1.a) <span class="hljs-keyword">FROM</span> t t1; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="limit_to_cop" style="position:relative">LIMIT_TO_COP()<a href="#limit_to_cop" aria-label="limit_to_cop permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>LIMIT_TO_COP()</code> 提示优化器将指定查询块中的 <code>Limit</code> 和 <code>TopN</code> 算子下推到 coprocessor。优化器没有下推 <code>Limit</code> 或者 <code>TopN</code> 算子时建议尝试使用该提示。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ LIMIT_TO_COP() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> a <span class="hljs-operator">=</span> <span class="hljs-number">1</span> <span class="hljs-keyword">AND</span> b <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">ORDER</span> <span class="hljs-keyword">BY</span> c LIMIT <span class="hljs-number">1</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="read_from_storagetiflasht1_name--tl_name--tikvt2_name--tl_name-" style="position:relative">READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]])<a href="#read_from_storagetiflasht1_name--tl_name--tikvt2_name--tl_name-" aria-label="read_from_storagetiflasht1_name tl_name tikvt2_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]])</code> 提示优化器从指定的存储引擎来读取指定的表，目前支持的存储引擎参数有 <code>TIKV</code> 和 <code>TIFLASH</code>。如果为表指定了别名，就只能使用表的别名作为 <code>READ_FROM_STORAGE()</code> 的参数；如果没有指定别名，则用表的本名作为其参数。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ READ_FROM_STORAGE(TIFLASH[t1], TIKV[t2]) */</span> t1.a <span class="hljs-keyword">FROM</span> t t1, t t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t2.a; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="use_index_merget1_name-idx1_name--idx2_name-" style="position:relative">USE_INDEX_MERGE(t1_name, idx1_name [, idx2_name ...])<a href="#use_index_merget1_name-idx1_name--idx2_name-" aria-label="use_index_merget1_name idx1_name idx2_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>USE_INDEX_MERGE(t1_name, idx1_name [, idx2_name ...])</code> 提示优化器通过索引合并的方式来访问指定的表。索引合并分为并集型和交集型两种类型，详情参见<a href="/zh/tidb/stable/explain-index-merge/">用 EXPLAIN 查看索引合并的 SQL 执行计划</a>。</p><p>若显式地指定索引列表，优化器会尝试在索引列表中选取索引来构建索引合并。若不指定索引列表，优化器会尝试在所有可用的索引中选取索引来构建索引合并。</p><p>对于交集型索引合并，索引列表是必选参数。对于并集型索引合并，Hint 中的索引列表为可选参数。示例如下。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ USE_INDEX_MERGE(t1, idx_a, idx_b, idx_c) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span> <span class="hljs-keyword">OR</span> t1.b <span class="hljs-operator">&gt;</span> <span class="hljs-number">10</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>当对同一张表有多个 <code>USE_INDEX_MERGE</code> Hint 时，优化器会从这些 Hint 指定的索引列表的并集中尝试选取索引。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>USE_INDEX_MERGE</code> 的参数是索引名，而不是列名。对于主键索引，索引名为 <code>primary</code>。</p></div></div></div><h3 id="leadingt1_name--tl_name-" style="position:relative">LEADING(t1_name [, tl_name ...])<a href="#leadingt1_name--tl_name-" aria-label="leadingt1_name tl_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>LEADING(t1_name [, tl_name ...])</code> 提示优化器在生成多表连接的执行计划时，按照 hint 中表名出现的顺序来确定多表连接的顺序。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ LEADING(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2, t3 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id <span class="hljs-keyword">and</span> t2.id <span class="hljs-operator">=</span> t3.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>在以上多表连接查询语句中，<code>LEADING()</code> 中表出现的顺序决定了优化器将会先对表 <code>t1</code> 和 <code>t2</code> 进行连接，再将结果和表 <code>t3</code> 进行连接。该 hint 比 <a href="#straight_join"><code>STRAIGHT_JOIN</code></a> 更为通用。</p><p><code>LEADING</code> hint 在以下情况下会失效：</p><ul><li>指定了多个 <code>LEADING</code> hint</li><li><code>LEADING</code> hint 中指定的表名不存在</li><li><code>LEADING</code> hint 中指定了重复的表名</li><li>优化器无法按照 <code>LEADING</code> hint 指定的顺序进行表连接</li><li>已经存在 <code>straight_join()</code> hint</li><li>查询语句中包含 outer join 且同时指定了包含笛卡尔积的情况</li></ul><p>当出现了上述失效的情况，会输出 warning 警告。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 指定了多个 LEADING hint</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ LEADING(t1, t2) LEADING(t3) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2, t3 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id <span class="hljs-keyword">and</span> t2.id <span class="hljs-operator">=</span> t3.id; <span class="hljs-comment">-- 通过执行 `show warnings` 了解具体产生冲突的原因</span> <span class="hljs-keyword">SHOW</span> WARNINGS; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Level <span class="hljs-operator">|</span> Code <span class="hljs-operator">|</span> Message <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Warning <span class="hljs-operator">|</span> <span class="hljs-number">1815</span> <span class="hljs-operator">|</span> We can <span class="hljs-keyword">only</span> use <span class="hljs-keyword">one</span> <span class="hljs-keyword">leading</span> hint <span class="hljs-keyword">at</span> most, <span class="hljs-keyword">when</span> multiple <span class="hljs-keyword">leading</span> hints <span class="hljs-keyword">are</span> used, <span class="hljs-keyword">all</span> <span class="hljs-keyword">leading</span> hints will be invalid <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>如果查询语句中包含了 outer join，你只能在 hint 中指定可以用于交换连接顺序的表。如果 hint 中存在不能用于交换的表，则该 hint 会失效。例如在 <code>SELECT * FROM t1 LEFT JOIN (t2 JOIN t3 JOIN t4) ON t1.a = t2.a;</code> 中，如果想要控制 <code>t2</code>、<code>t3</code>、<code>t4</code> 表的连接顺序，那么在使用 <code>LEADING</code> hint 时，hint 中不能出现 <code>t1</code> 表。</p></div></div></div><h3 id="merge" style="position:relative">MERGE()<a href="#merge" aria-label="merge permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>在含有<a href="/zh/tidb/stable/dev-guide-use-common-table-expression/">公共表表达式</a>的查询中使用 <code>MERGE()</code> hint，可关闭对当前子查询的物化过程，并将内部查询的内联展开到外部查询。该 hint 适用于非递归的公共表表达式查询，在某些场景下，使用该 hint 会比默认分配一块临时空间的语句执行效率更高。例如将外部查询的条件下推或在嵌套的 CTE 查询中：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-comment">-- 使用 hint 将外部查询条件的谓词下推</span> <span class="hljs-keyword">WITH</span> CTE <span class="hljs-keyword">AS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MERGE() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> tc <span class="hljs-keyword">WHERE</span> tc.a <span class="hljs-operator">&lt;</span> <span class="hljs-number">60</span>) <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> CTE <span class="hljs-keyword">WHERE</span> CTE.a <span class="hljs-operator">&lt;</span><span class="hljs-number">18</span>; <span class="hljs-comment">-- 在嵌套 CTE 查询中使用该 hint 来指定将某个 CTE 内联展开到外部查询</span> <span class="hljs-keyword">WITH</span> CTE1 <span class="hljs-keyword">AS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1), CTE2 <span class="hljs-keyword">AS</span> (<span class="hljs-keyword">WITH</span> CTE3 <span class="hljs-keyword">AS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MERGE() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t2), CTE4 <span class="hljs-keyword">AS</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t3) <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> CTE3, CTE4) <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> CTE1, CTE2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>MERGE()</code> 只适用于简单的 CTE 查询，在以下情况下无法使用该 hint：</p><ul><li><a href="/zh/tidb/stable/dev-guide-use-common-table-expression/#递归的-cte">递归的 CTE 查询</a></li><li>子查询中有无法进行内联展开的部分，例如聚合算子、窗口函数以及 <code>DINSTINCT</code> 等</li></ul><p>当 CTE 引用次数过多时，查询性能可能低于默认的物化方式。</p></div></div></div><h2 id="全局生效的-hint" style="position:relative">全局生效的 Hint<a href="#%E5%85%A8%E5%B1%80%E7%94%9F%E6%95%88%E7%9A%84-hint" aria-label="全局生效的 hint permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h2><p>全局生效的 Hint 和<a href="/zh/tidb/stable/views/">视图</a>有关，可以使查询中定义的 Hint 能够在视图内部生效。添加这类 Hint 需要两步：先用 <code>QB_NAME</code> Hint 为视图内的查询块命名，再以“视图名@查询块名”的方式加入实际需要的 Hint。</p><h3 id="第-1-步使用-qb_name-hint-重命名视图内的查询块" style="position:relative">第 1 步：使用 <code>QB_NAME</code> Hint 重命名视图内的查询块<a href="#%E7%AC%AC-1-%E6%AD%A5%E4%BD%BF%E7%94%A8-qb_name-hint-%E9%87%8D%E5%91%BD%E5%90%8D%E8%A7%86%E5%9B%BE%E5%86%85%E7%9A%84%E6%9F%A5%E8%AF%A2%E5%9D%97" aria-label="第 1 步使用 qb_name hint 重命名视图内的查询块 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>首先使用 <a href="#qb_name"><code>QB_NAME</code> Hint</a> 重命名视图内部的查询块。其中针对视图的 <code>QB_NAME</code> Hint 的概念与<a href="#qb_name">查询块范围生效的 <code>QB_NAME</code> Hint</a>相同，只是在语法上进行了相应的拓展。从 <code>QB_NAME(QB)</code> 拓展为 <code>QB_NAME(QB, 视图名@查询块名 [.视图名@查询块名 .视图名@查询块名 ...])</code>。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>@查询块名</code> 与后面紧跟的 <code>.视图名@查询块名</code> 部分之间需要有一个空格，否则 <code>.视图名@查询块名</code> 会被视作前面 <code>@查询块名</code> 的一部分。例如，<code>QB_NAME(v2_1, v2@SEL_1 .@SEL_1)</code> 不能写为 <code>QB_NAME(v2_1, v2@SEL_1.@SEL_1)</code>。</p></div></div></div><ul><li><p>对于单个视图、不包含子查询的简单语句，下面以重命名视图 <code>v</code> 的第一个查询块为例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/* 注释：当前查询块的名字为默认的 @SEL_1 */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p> 对于视图 <code>v</code> 来说，从查询语句开始的首个视图是 <code>v@SEL_1</code>。视图 <code>v</code> 的第一个查询块可以声明为 <code>QB_NAME(v_1, v@SEL_1 .@SEL_1)</code>，也可以省略 <code>@SEL_1</code> 简写成 <code>QB_NAME(v_1, v)</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">VIEW</span> v <span class="hljs-keyword">AS</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/* 注释：当前查询块的名字为默认的 @SEL_1 */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t; <span class="hljs-comment">-- 使用全局生效的 Hint</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(v_1, v) USE_INDEX(t@v_1, idx) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li><li><p>对于嵌套视图和包含子查询的复杂语句，下面以重命名视图 <code>v1</code>、<code>v2</code> 的两个查询块为例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/* 注释：当前查询块的名字为默认的 @SEL_1 */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2 <span class="hljs-keyword">JOIN</span> ( <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/* 注释：当前查询块的名字为默认的 @SEL_2 */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2) vv; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p> 对于第一个视图 <code>v2</code> 来说，从上面的语句开始的首个视图是 <code>v2@SEL_1</code>。对于第二个视图 <code>v2</code> 来说，首个视图表为 <code>v2@SEL_2</code>。下面的查询部分仅考虑第一个视图 <code>v2</code>。</p><p> 视图 <code>v2</code> 的第一个查询块可以声明为 <code>QB_NAME(v2_1, v2@SEL_1 .@SEL_1)</code>，视图 <code>v2</code> 的第二个查询块可以声明为 <code>QB_NAME(v2_2, v2@SEL_1 .@SEL_2)</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">VIEW</span> v2 <span class="hljs-keyword">AS</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">JOIN</span> <span class="hljs-comment">/* 注释：对于视图 v2 来说，当前查询块的名字为默认的 @SEL_1，因此当前查询块的视图列表是 v2@SEL_1 .@SEL_1 */</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-built_in">COUNT</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">JOIN</span> v1 <span class="hljs-comment">/* 注释：对于视图 v2 来说，当前查询块的名字为默认的 @SEL_2，因此当前查询块的视图列表是 v2@SEL_1 .@SEL_2 */</span> ) tt; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p> 对于视图 <code>v1</code> 来说，从上面的语句开始的首个视图是 <code>v2@SEL_1 .v1@SEL_2</code>。视图 <code>v1</code> 的第一个查询块可以声明为 <code>QB_NAME(v1_1, v2@SEL_1 .v1@SEL_2 .@SEL_1)</code>，视图 <code>v1</code> 的第二个查询块可以声明为 <code>QB_NAME(v1_2, v2@SEL_1 .v1@SEL_2 .@SEL_2)</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">VIEW</span> v1 <span class="hljs-keyword">AS</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">JOIN</span> <span class="hljs-comment">/* 注释：对于视图 v1 来说，当前查询块的名字为默认的 @SEL_1，因此当前查询块的视图列表是 v2@SEL_1 .v1@SEL_2 .@SEL_1 */</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-built_in">COUNT</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">JOIN</span> t2 <span class="hljs-comment">/* 注释：对于视图 v1 来说，当前查询块的名字为默认的 @SEL_2，因此当前查询块的视图列表是 v2@SEL_1 .v1@SEL_2 .@SEL_2 */</span> ) tt; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li></ul><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li><p>与视图相关的全局生效的 Hint 必须先定义了对应的 <code>QB_NAME</code> Hint 才能使用。</p></li><li><p>使用一个 Hint 来指定视图内的多个表名时，需要保证在同一个 Hint 中出现的表名处于同一个视图的同一个查询块中。</p></li><li><p>对于最外层的查询来说，在定义和视图相关的 <code>QB_NAME</code> Hint 时：</p><ul><li>对于 <code>QB_NAME</code> Hint 中视图列表序列的第一项，在不显式声明 <code>@SEL_</code> 时，默认和定义 <code>QB_NAME</code> Hint 的查询块位置保持一致，即省略 <code>@SEL_</code> 的查询 <code>SELECT /*+ QB_NAME(qb1, v2) */ * FROM v2 JOIN (SELECT /*+ QB_NAME(qb2, v2) */ * FROM v2) vv;</code> 相当于 <code>SELECT /*+ QB_NAME(qb1, v2@SEL_1) */ * FROM v2 JOIN (SELECT /*+ QB_NAME(qb2, v2@SEL_2) */ * FROM v2) vv;</code>。</li><li>对于 <code>QB_NAME</code> Hint 中视图列表序列第一项之外的其他部分，只有 <code>@SEL_1</code> 可省略。即，如果声明处于当前部分的第一个查询块中，则 <code>@SEL_1</code> 可以省略，否则，不能省略 <code>@SEL_</code>。对于上面的例子：<ul><li>视图 <code>v2</code> 的第一个查询块可以声明为 <code>QB_NAME(v2_1, v2)</code></li><li>视图 <code>v2</code> 的第二个查询块可以声明为 <code>QB_NAME(v2_2, v2.@SEL_2)</code></li><li>视图 <code>v1</code> 的第一个查询块可以声明为 <code>QB_NAME(v1_1, v2.v1@SEL_2)</code></li><li>视图 <code>v1</code> 的第二个查询块可以声明为 <code>QB_NAME(v1_2, v2.v1@SEL_2 .@SEL_2)</code></li></ul></li></ul></li></ul></div></div></div><h3 id="第-2-步添加实际需要的-hint" style="position:relative">第 2 步：添加实际需要的 Hint<a href="#%E7%AC%AC-2-%E6%AD%A5%E6%B7%BB%E5%8A%A0%E5%AE%9E%E9%99%85%E9%9C%80%E8%A6%81%E7%9A%84-hint" aria-label="第 2 步添加实际需要的 hint permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>在定义好视图查询块部分的 <code>QB_NAME</code> Hint 后，你可以通过查询块的名字使用<a href="#%E6%9F%A5%E8%AF%A2%E5%9D%97%E8%8C%83%E5%9B%B4%E7%94%9F%E6%95%88%E7%9A%84-hint">查询块范围生效的 Hint</a>，以“视图名@查询块名”的方式加入实际需要的 Hint，使其在视图内部生效。例如：</p><ul><li><p>指定视图 <code>v2</code> 中第一个查询块的 <code>MERGE_JOIN()</code> Hint：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(v2_1, v2) merge_join(t@v2_1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li><li><p>指定视图 <code>v2</code> 中第二个查询块的 <code>MERGE_JOIN()</code> 和 <code>STREAM_AGG()</code> Hint：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(v2_2, v2.@SEL_2) merge_join(t1@v2_2) stream_agg(@v2_2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li><li><p>指定视图 <code>v1</code> 中第一个查询块的 <code>HASH_JOIN()</code> Hint：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(v1_1, v2.v1@SEL_2) hash_join(t@v1_1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li><li><p>指定视图 <code>v1</code> 中第二个查询块的 <code>HASH_JOIN()</code> 和 <code>HASH_AGG()</code> Hint：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ QB_NAME(v1_2, v2.v1@SEL_2 .@SEL_2) hash_join(t1@v1_2) hash_agg(@v1_2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> v2; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre></li></ul><h2 id="查询范围生效的-hint" style="position:relative">查询范围生效的 Hint<a href="#%E6%9F%A5%E8%AF%A2%E8%8C%83%E5%9B%B4%E7%94%9F%E6%95%88%E7%9A%84-hint" aria-label="查询范围生效的 hint permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h2><p>这类 Hint 只能跟在语句中<strong>第一个</strong> <code>SELECT</code>、<code>UPDATE</code> 或 <code>DELETE</code> 关键字的后面，等同于在当前这条查询运行时对指定的系统变量进行修改，其优先级高于现有系统变量的值。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>这类 Hint 虽然也有隐藏的可选变量 <code>@QB_NAME</code>，但就算指定了该值，Hint 还是会在整个查询范围生效。</p></div></div></div><h3 id="no_index_merge" style="position:relative">NO_INDEX_MERGE()<a href="#no_index_merge" aria-label="no_index_merge permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NO_INDEX_MERGE()</code> 会关闭优化器的 index merge 功能。</p><p>下面的例子不会使用 index merge：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_INDEX_MERGE() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> t.a <span class="hljs-operator">&gt;</span> <span class="hljs-number">0</span> <span class="hljs-keyword">or</span> t.b <span class="hljs-operator">&gt;</span> <span class="hljs-number">0</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>除了 Hint 外，系统变量 <code>tidb_enable_index_merge</code> 也能决定是否开启该功能。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li><code>NO_INDEX_MERGE</code> 优先级高于 <code>USE_INDEX_MERGE</code>，当这两类 Hint 同时存在时，<code>USE_INDEX_MERGE</code> 不会生效。</li><li>当存在子查询时，<code>NO_INDEX_MERGE</code> 放在最外层才能生效。</li></ul></div></div></div><h3 id="use_tojaboolean_value" style="position:relative">USE_TOJA(boolean_value)<a href="#use_tojaboolean_value" aria-label="use_tojaboolean_value permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>参数 <code>boolean_value</code> 可以是 <code>TRUE</code> 或者 <code>FALSE</code>。<code>USE_TOJA(TRUE)</code> 会开启优化器尝试将 in (subquery) 条件转换为 join 和 aggregation 的功能。相对地，<code>USE_TOJA(FALSE)</code> 会关闭该功能。</p><p>下面的例子会将 <code>in (SELECT t2.a FROM t2) subq</code> 转换为等价的 join 和 aggregation：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ USE_TOJA(TRUE) */</span> t1.a, t1.b <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-keyword">in</span> (<span class="hljs-keyword">SELECT</span> t2.a <span class="hljs-keyword">FROM</span> t2) subq; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>除了 Hint 外，系统变量 <code>tidb_opt_insubq_to_join_and_agg</code> 也能决定是否开启该功能。</p><h3 id="max_execution_timen" style="position:relative">MAX_EXECUTION_TIME(N)<a href="#max_execution_timen" aria-label="max_execution_timen permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>MAX_EXECUTION_TIME(N)</code> 把语句的执行时间限制在 <code>N</code> 毫秒以内，超时后服务器会终止这条语句的执行。</p><p>下面的 Hint 设置了 1000 毫秒（即 1 秒）超时：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MAX_EXECUTION_TIME(1000) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span> t2 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>除了 Hint 之外，系统变量 <code>global.max_execution_time</code> 也能对语句执行时间进行限制。</p><h3 id="memory_quotan" style="position:relative">MEMORY_QUOTA(N)<a href="#memory_quotan" aria-label="memory_quotan permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>MEMORY_QUOTA(N)</code> 用于限制语句执行时的内存使用。该 Hint 支持 MB 和 GB 两种单位。内存使用超过该限制时会根据当前设置的内存超限行为来打出一条 log 或者终止语句的执行。</p><p>下面的 Hint 设置了 1024 MB 的内存限制：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ MEMORY_QUOTA(1024 MB) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>除了 Hint 外，系统变量 <a href="/zh/tidb/stable/system-variables/#tidb_mem_quota_query"><code>tidb_mem_quota_query</code></a> 也能限制语句执行的内存使用。</p><h3 id="read_consistent_replica" style="position:relative">READ_CONSISTENT_REPLICA()<a href="#read_consistent_replica" aria-label="read_consistent_replica permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>READ_CONSISTENT_REPLICA()</code> 会开启从数据一致的 TiKV follower 节点读取数据的特性。</p><p>下面的例子会从 follower 节点读取数据：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ READ_CONSISTENT_REPLICA() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>除了 Hint 外，环境变量 <code>tidb_replica_read</code> 设为 <code>'follower'</code> 或者 <code>'leader'</code> 也能决定是否开启该特性。</p><h3 id="ignore_plan_cache" style="position:relative">IGNORE_PLAN_CACHE()<a href="#ignore_plan_cache" aria-label="ignore_plan_cache permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>IGNORE_PLAN_CACHE()</code> 提示优化器在处理当前 <code>prepare</code> 语句时不使用 plan cache。</p><p>该 Hint 用于在 <a href="/zh/tidb/stable/sql-prepared-plan-cache/">Prepared Plan Cache</a> 开启的场景下临时对某类查询禁用 plan cache。</p><p>以下示例强制该 <code>prepare</code> 语句不使用 plan cache：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">FROM</span> <span class="hljs-string">'SELECT /*+ IGNORE_PLAN_CACHE() */ * FROM t WHERE t.id = ?'</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="set_varvar_namevar_value" style="position:relative">SET_VAR(VAR_NAME=VAR_VALUE)<a href="#set_varvar_namevar_value" aria-label="set_varvar_namevar_value permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>SET_VAR(VAR_NAME=VAR_VALUE)</code> 允许在语句执行期间以 Hint 形式临时修改会话级系统变量的值。当语句执行完成后，系统变量将在当前会话中自动恢复为原始值。通过这个 Hint 可以修改一部分与优化器、执行器相关的系统变量行为。支持通过 <code>SET_VAR(VAR_NAME=VAR_VALUE)</code> Hint 修改的系统变量请查看<a href="/zh/tidb/stable/system-variables/">系统变量</a>。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardError MuiAlert-standard css-kdwypz" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ErrorOutlineIcon"><path d="M11 15h2v2h-2zm0-8h2v6h-2zm.99-5C6.47 2 2 6.48 2 12s4.47 10 9.99 10C17.52 22 22 17.52 22 12S17.52 2 11.99 2zM12 20c-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8-3.58 8-8 8z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">警告</div><ul><li>强烈建议不要利用此 Hint 修改没有明确支持的变量，这可能会引发不可预知的行为。</li><li>注意不要把 <code>SET_VAR</code> 写在子查询中，否则可能会不生效。详情请参考 <a href="#set_var-%E5%86%99%E5%9C%A8%E5%AD%90%E6%9F%A5%E8%AF%A2%E4%B8%AD%E4%B8%8D%E7%94%9F%E6%95%88"><code>SET_VAR</code> 写在子查询中不生效</a>。</li></ul></div></div></div><p>下面是一个使用示例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ SET_VAR(MAX_EXECUTION_TIME=1234) */</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME; <span class="hljs-keyword">SELECT</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>执行上述 SQL，第一个查询返回的结果是 Hint 中设置的 <code>1234</code>，而不是变量 <code>MAX_EXECUTION_TIME</code> 的默认值。第二个查询会返回变量的默认值。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-operator">|</span> <span class="hljs-number">1234</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-operator">|</span> <span class="hljs-number">0</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="straight_join" style="position:relative">STRAIGHT_JOIN()<a href="#straight_join" aria-label="straight_join permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>STRAIGHT_JOIN()</code> 提示优化器在生成表连接顺序时按照表名在 <code>FROM</code> 子句中出现的顺序进行连接。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ STRAIGHT_JOIN() */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t t1, t t2 <span class="hljs-keyword">WHERE</span> t1.a <span class="hljs-operator">=</span> t2.a; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><ul><li><code>STRAIGHT_JOIN</code> 优先级高于 <code>LEADING</code>，当这两类 Hint 同时存在时，<code>LEADING</code> 不会生效。</li><li>建议使用 <code>LEADING</code> Hint，它比 <code>STRAIGHT_JOIN</code> Hint 更通用。</li></ul></div></div></div><h3 id="nth_plann" style="position:relative">NTH_PLAN(N)<a href="#nth_plann" aria-label="nth_plann permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>NTH_PLAN(N)</code> 提示优化器选用在物理优化阶段搜索到的第 <code>N</code> 个物理计划。<code>N</code> 必须是正整数。</p><p>如果指定的 <code>N</code> 超出了物理优化阶段的搜索范围，TiDB 会返回 warning，并根据不存在该 Hint 时一样的策略选择最优物理计划。</p><p>该 Hint 在启用 cascades planner 的情况下不会生效。</p><p>以下示例会强制优化器在物理阶段选择搜索到的第 3 个物理计划：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NTH_PLAN(3) */</span> <span class="hljs-built_in">count</span>(<span class="hljs-operator">*</span>) <span class="hljs-keyword">from</span> t <span class="hljs-keyword">where</span> a <span class="hljs-operator">&gt;</span> <span class="hljs-number">5</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p><code>NTH_PLAN(N)</code> 主要用于测试用途，并且在未来不保证其兼容性，请谨慎使用。</p></div></div></div><h3 id="resource_groupresource_group_name" style="position:relative">RESOURCE_GROUP(resource_group_name)<a href="#resource_groupresource_group_name" aria-label="resource_groupresource_group_name permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>RESOURCE_GROUP(resource_group_name)</code> 用于<a href="/zh/tidb/stable/tidb-resource-control-ru-groups/">使用资源管控 (Resource Control) 实现资源组限制和流控</a>。此 Hint 将临时使用指定的资源组执行当前的语句。如果指定的资源组不存在，则该 Hint 将被忽略。</p><p>示例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ RESOURCE_GROUP(rg1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t limit <span class="hljs-number">10</span>; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-1bzshic" role="alert"><div class="MuiAlert-icon css-1l54tgj"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><div class="MuiAlert-message css-1xsto0d"><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-m00e13">注意</div><p>自 v8.2.0 版本开始，TiDB 为此 Hint 引入权限控制。当系统变量 <a href="/zh/tidb/stable/system-variables/#tidb_resource_control_strict_mode-从-v820-版本开始引入"><code>tidb_resource_control_strict_mode</code></a> 设置为 <code>ON</code> 时，你需要有 <code>SUPER</code> 或者 <code>RESOURCE_GROUP_ADMIN</code> 或者 <code>RESOURCE_GROUP_USER</code> 权限才能使用此 Hint。如果没有所需权限，则此 Hint 会被忽略，同时 TiDB 会返回 warning，你可以在查询结束后通过 <code>SHOW WARNINGS;</code> 命令查看具体信息。</p></div></div></div><h2 id="常见-hint-不生效问题排查" style="position:relative">常见 Hint 不生效问题排查<a href="#%E5%B8%B8%E8%A7%81-hint-%E4%B8%8D%E7%94%9F%E6%95%88%E9%97%AE%E9%A2%98%E6%8E%92%E6%9F%A5" aria-label="常见 hint 不生效问题排查 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h2><h3 id="mysql-命令行客户端清除-hint-导致不生效" style="position:relative">MySQL 命令行客户端清除 Hint 导致不生效<a href="#mysql-%E5%91%BD%E4%BB%A4%E8%A1%8C%E5%AE%A2%E6%88%B7%E7%AB%AF%E6%B8%85%E9%99%A4-hint-%E5%AF%BC%E8%87%B4%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="mysql 命令行客户端清除 hint 导致不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>MySQL 命令行客户端在 5.7.7 版本之前默认清除了 Optimizer Hints。如果需要在这些早期版本的客户端中使用 Hint 语法，需要在启动客户端时加上 <code>--comments</code> 选项。例如 <code>mysql -h 127.0.0.1 -P 4000 -uroot --comments</code>。</p><h3 id="创建连接时不指定库名导致-hint-不生效" style="position:relative">创建连接时不指定库名导致 Hint 不生效<a href="#%E5%88%9B%E5%BB%BA%E8%BF%9E%E6%8E%A5%E6%97%B6%E4%B8%8D%E6%8C%87%E5%AE%9A%E5%BA%93%E5%90%8D%E5%AF%BC%E8%87%B4-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="创建连接时不指定库名导致 hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>如果创建连接时未指定数据库名，则可能出现 Hint 失效的情况。例如：</p><p>使用 <code>mysql -h127.0.0.1 -P4000 -uroot</code> 命令连接数据库时，未使用 <code>-D</code> 参数指定数据库名。然后执行下面的 SQL 语句：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ use_index(t, a) */</span> a <span class="hljs-keyword">FROM</span> test.t; <span class="hljs-keyword">SHOW</span> WARNINGS; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>由于无法识别表 <code>t</code> 对应的数据库名，因此 <code>use_index(t, a)</code> Hint 无法生效。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Level <span class="hljs-operator">|</span> Code <span class="hljs-operator">|</span> Message <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Warning <span class="hljs-operator">|</span> <span class="hljs-number">1815</span> <span class="hljs-operator">|</span> use_index(.t, a) <span class="hljs-keyword">is</span> inapplicable, <span class="hljs-keyword">check</span> whether the <span class="hljs-keyword">table</span>(.t) <span class="hljs-keyword">exists</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="跨库查询不指定库名导致-hint-不生效" style="position:relative">跨库查询不指定库名导致 Hint 不生效<a href="#%E8%B7%A8%E5%BA%93%E6%9F%A5%E8%AF%A2%E4%B8%8D%E6%8C%87%E5%AE%9A%E5%BA%93%E5%90%8D%E5%AF%BC%E8%87%B4-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="跨库查询不指定库名导致 hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>对于跨库查询中需要访问的表，需要显式地指定数据库名，否则可能出现 Hint 失效的情况。例如执行下面跨库查询的 SQL 语句：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">USE test1; <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1(a <span class="hljs-type">INT</span>, KEY(a)); USE test2; <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2(a <span class="hljs-type">INT</span>, KEY(a)); <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ use_index(t1, a) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> test1.t1, t2; <span class="hljs-keyword">SHOW</span> WARNINGS; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>由于 <code>t1</code> 不在当前数据库 <code>test2</code> 下，因此 <code>use_index(t1, a)</code> Hint 无法被正确地识别。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Level <span class="hljs-operator">|</span> Code <span class="hljs-operator">|</span> Message <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Warning <span class="hljs-operator">|</span> <span class="hljs-number">1815</span> <span class="hljs-operator">|</span> use_index(test2.t1, a) <span class="hljs-keyword">is</span> inapplicable, <span class="hljs-keyword">check</span> whether the <span class="hljs-keyword">table</span>(test2.t1) <span class="hljs-keyword">exists</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>此时，需要显式地指定库名，即将 <code>use_index(t1, a)</code> 修改为 <code>use_index(test1.t1, a)</code>。</p><h3 id="hint-位置不正确导致不生效" style="position:relative">Hint 位置不正确导致不生效<a href="#hint-%E4%BD%8D%E7%BD%AE%E4%B8%8D%E6%AD%A3%E7%A1%AE%E5%AF%BC%E8%87%B4%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="hint 位置不正确导致不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>如果没有按照 Optimizer Hints 语法将 Hint 正确地放在指定关键字的后面，它将无法生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-comment">/*+ use_index(t, a) */</span> <span class="hljs-keyword">FROM</span> t; <span class="hljs-keyword">SHOW</span> WARNINGS; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>Warning 信息如下：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Level <span class="hljs-operator">|</span> Code <span class="hljs-operator">|</span> Message <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Warning <span class="hljs-operator">|</span> <span class="hljs-number">1064</span> <span class="hljs-operator">|</span> You have an error <span class="hljs-keyword">in</span> your <span class="hljs-keyword">SQL</span> syntax; <span class="hljs-keyword">check</span> the manual that corresponds <span class="hljs-keyword">to</span> your TiDB version <span class="hljs-keyword">for</span> the <span class="hljs-keyword">right</span> syntax <span class="hljs-keyword">to</span> use [parser:<span class="hljs-number">8066</span>]Optimizer hint can <span class="hljs-keyword">only</span> be followed <span class="hljs-keyword">by</span> certain keywords <span class="hljs-keyword">like</span> <span class="hljs-keyword">SELECT</span>, <span class="hljs-keyword">INSERT</span>, etc. <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.01</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>在上面的示例中，你需要将 Hint 直接放在 <code>SELECT</code> 关键字之后。具体的语法规则参见 <a href="#%E8%AF%AD%E6%B3%95">Hint 语法</a>部分。</p><h3 id="inl_join-hint-不生效" style="position:relative"><code>INL_JOIN</code> Hint 不生效<a href="#inl_join-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="inl_join hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><h4 id="关联表的列上使用内置函数导致-inl_join-hint-不生效" style="position:relative">关联表的列上使用内置函数导致 <code>INL_JOIN</code> Hint 不生效<a href="#%E5%85%B3%E8%81%94%E8%A1%A8%E7%9A%84%E5%88%97%E4%B8%8A%E4%BD%BF%E7%94%A8%E5%86%85%E7%BD%AE%E5%87%BD%E6%95%B0%E5%AF%BC%E8%87%B4-inl_join-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="关联表的列上使用内置函数导致 inl_join hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h4><p>在某些情况下，如果在关联表的列上使用了内置函数，优化器可能无法选择 <code>IndexJoin</code> 计划，导致 <code>INL_JOIN</code> Hint 也无法生效。</p><p>例如，以下查询在关联表的列 <code>tname</code> 上使用了内置函数 <code>substr</code>：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1 (id <span class="hljs-type">varchar</span>(<span class="hljs-number">10</span>) <span class="hljs-keyword">primary</span> key, tname <span class="hljs-type">varchar</span>(<span class="hljs-number">10</span>)); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2 (id <span class="hljs-type">varchar</span>(<span class="hljs-number">10</span>) <span class="hljs-keyword">primary</span> key, tname <span class="hljs-type">varchar</span>(<span class="hljs-number">10</span>)); EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ INL_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id<span class="hljs-operator">=</span>t2.id <span class="hljs-keyword">and</span> SUBSTR(t1.tname,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>)<span class="hljs-operator">=</span>SUBSTR(t2.tname,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>查询计划输出结果如下：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_12 <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, equal:[eq(test.t1.id, test.t2.id) eq(<span class="hljs-keyword">Column</span>#<span class="hljs-number">5</span>, <span class="hljs-keyword">Column</span>#<span class="hljs-number">6</span>)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─Projection_17(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t2.id, test.t2.tname, substr(test.t2.tname, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">6</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableReader_19 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_18 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_18 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Projection_14(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t1.id, test.t1.tname, substr(test.t1.tname, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">5</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_16 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_15 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_15 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+----------+-----------+---------------+-----------------------------------------------------------------------+</span> <span class="hljs-number">7</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span>, <span class="hljs-number">1</span> warning (<span class="hljs-number">0.01</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SHOW</span> WARNINGS; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-plaintext"><div class="MuiBox-root css-1qhimia">+---------+------+------------------------------------------------------------------------------------+ | Level | Code | Message | +---------+------+------------------------------------------------------------------------------------+ | Warning | 1815 | Optimizer Hint /*+ INL_JOIN(t1, t2) */ or /*+ TIDB_INLJ(t1, t2) */ is inapplicable | +---------+------+------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>从该示例中可以看到，<code>INL_JOIN</code> Hint 没有生效。该问题的根本原因是优化器限制导致无法使用 <code>Projection</code> 或者 <code>Selection</code> 算子作为 <code>IndexJoin</code> 的探测 (Probe) 端。</p><p>从 TiDB v8.0.0 起，你通过设置 <a href="/zh/tidb/stable/system-variables/#tidb_enable_inl_join_inner_multi_pattern-从-v700-版本开始引入"><code>tidb_enable_inl_join_inner_multi_pattern</code></a> 为 <code>ON</code> 来避免该问题。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SET</span> @<span class="hljs-variable">@tidb</span>_enable_inl_join_inner_multi_pattern<span class="hljs-operator">=</span><span class="hljs-keyword">ON</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ INL_JOIN(t1, t2) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.id<span class="hljs-operator">=</span>t2.id <span class="hljs-keyword">AND</span> SUBSTR(t1.tname,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>)<span class="hljs-operator">=</span>SUBSTR(t2.tname,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>); <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_18 <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:Projection_14, <span class="hljs-keyword">outer</span> key:test.t1.id, <span class="hljs-keyword">inner</span> key:test.t2.id, equal cond:eq(<span class="hljs-keyword">Column</span>#<span class="hljs-number">5</span>, <span class="hljs-keyword">Column</span>#<span class="hljs-number">6</span>), eq(test.t1.id, test.t2.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─Projection_32(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t1.id, test.t1.tname, substr(test.t1.tname, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">5</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableReader_34 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Projection_14(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">100000000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t2.id, test.t2.tname, substr(test.t2.tname, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span><span class="hljs-keyword">Column</span>#<span class="hljs-number">6</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_13 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_12 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_12 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [eq(test.t2.id, test.t1.id)], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+--------------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-number">7</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h4 id="排序规则不兼容导致-inl_join-hintinl_hash_join-hintinl_merge_join-hint-不生效" style="position:relative">排序规则不兼容导致 <code>INL_JOIN</code> Hint、<code>INL_HASH_JOIN</code> Hint、<code>INL_MERGE_JOIN</code> Hint 不生效<a href="#%E6%8E%92%E5%BA%8F%E8%A7%84%E5%88%99%E4%B8%8D%E5%85%BC%E5%AE%B9%E5%AF%BC%E8%87%B4-inl_join-hintinl_hash_join-hintinl_merge_join-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="排序规则不兼容导致 inl_join hintinl_hash_join hintinl_merge_join hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h4><p>如果两个表的 Join key 的排序规则不能兼容，将无法使用 IndexJoin 来执行查询。此时 <a href="#inl_joint1_name--tl_name-"><code>INL_JOIN</code> Hint</a>、<a href="#inl_hash_join"><code>INL_HASH_JOIN</code> Hint</a>、<a href="#inl_merge_join"><code>INL_MERGE_JOIN</code> Hint</a> 将无法生效。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1 (k <span class="hljs-type">varchar</span>(<span class="hljs-number">8</span>), key(k)) <span class="hljs-keyword">COLLATE</span><span class="hljs-operator">=</span>utf8mb4_general_ci; <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2 (k <span class="hljs-type">varchar</span>(<span class="hljs-number">8</span>), key(k)) <span class="hljs-keyword">COLLATE</span><span class="hljs-operator">=</span>utf8mb4_bin; EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ tidb_inlj(t1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.k<span class="hljs-operator">=</span>t2.k; </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>查询计划输出结果如下：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+----------------------+----------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+----------------------+----------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_19 <span class="hljs-operator">|</span> <span class="hljs-number">12487.50</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, equal:[eq(test.t1.k, test.t2.k)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexReader_24(Build) <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_23 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_23 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2, index:k(k) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexReader_22(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_21 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexFullScan_21 <span class="hljs-operator">|</span> <span class="hljs-number">9990.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1, index:k(k) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+----------------------+----------------------------------------------+</span> <span class="hljs-number">5</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span>, <span class="hljs-number">1</span> warning (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>上面的 SQL 语句中 <code>t1.k</code> 和 <code>t2.k</code> 的排序规则不能相互兼容（分别为 <code>utf8mb4_general_ci</code> 和 <code>utf8mb4_bin</code>），导致 IndexJoin 无法适用。因此 <code>INL_JOIN</code> 或 <code>TIDB_INLJ</code> Hint 也无法生效。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">SHOW</span> WARNINGS; <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Level <span class="hljs-operator">|</span> Code <span class="hljs-operator">|</span> Message <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Warning <span class="hljs-operator">|</span> <span class="hljs-number">1815</span> <span class="hljs-operator">|</span> Optimizer Hint <span class="hljs-comment">/*+ INL_JOIN(t1) */</span> <span class="hljs-keyword">or</span> <span class="hljs-comment">/*+ TIDB_INLJ(t1) */</span> <span class="hljs-keyword">is</span> inapplicable <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------+------+----------------------------------------------------------------------------+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h4 id="连接顺序导致-inl_join-hint-不生效" style="position:relative">连接顺序导致 <code>INL_JOIN</code> Hint 不生效<a href="#%E8%BF%9E%E6%8E%A5%E9%A1%BA%E5%BA%8F%E5%AF%BC%E8%87%B4-inl_join-hint-%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="连接顺序导致 inl_join hint 不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h4><p><a href="#inl_joint1_name--tl_name-"><code>INL_JOIN(t1, t2)</code></a> 或 <code>TIDB_INLJ(t1, t2)</code> 的语义是让 <code>t1</code> 和 <code>t2</code> 作为 <code>IndexJoin</code> 的内表与其他表进行连接，而不是直接将 <code>t1</code> 和 <code>t2</code> 进行 <code>IndexJoin</code> 连接。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ inl_join(t1, t3) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2, t3 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id <span class="hljs-keyword">AND</span> t2.id <span class="hljs-operator">=</span> t3.id <span class="hljs-keyword">AND</span> t1.id <span class="hljs-operator">=</span> t3.id; <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_16 <span class="hljs-operator">|</span> <span class="hljs-number">15625.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_13, <span class="hljs-keyword">outer</span> key:test.t2.id, test.t1.id, <span class="hljs-keyword">inner</span> key:test.t3.id, test.t3.id, equal cond:eq(test.t1.id, test.t3.id), eq(test.t2.id, test.t3.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexJoin_34(Build) <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_31, <span class="hljs-keyword">outer</span> key:test.t2.id, <span class="hljs-keyword">inner</span> key:test.t1.id, equal cond:eq(test.t2.id, test.t1.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ ├─TableReader_40(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_39 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ │ └─TableFullScan_39 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableReader_31(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_30 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableRangeScan_30 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t2.id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_13(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_12 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_12 <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t3 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t2.id test.t1.id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>在上面例子中，<code>t1</code> 和 <code>t3</code> 并没有直接被一个 <code>IndexJoin</code> 连接起来。</p><p>如果想要直接使用 <code>IndexJoin</code> 来连接 <code>t1</code> 和 <code>t3</code>，需要先使用 <a href="#leadingt1_name--tl_name-"><code>LEADING</code> Hint</a> 指定 <code>t1</code> 和 <code>t3</code> 的连接顺序，然后再配合使用 <code>INL_JOIN</code>。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ leading(t1, t3), inl_join(t3) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2, t3 <span class="hljs-keyword">WHERE</span> t1.id <span class="hljs-operator">=</span> t2.id <span class="hljs-keyword">AND</span> t2.id <span class="hljs-operator">=</span> t3.id <span class="hljs-keyword">AND</span> t1.id <span class="hljs-operator">=</span> t3.id; <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> Projection_12 <span class="hljs-operator">|</span> <span class="hljs-number">15625.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> test.t1.id, test.t1.name, test.t2.id, test.t2.name, test.t3.id, test.t3.name <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─HashJoin_21 <span class="hljs-operator">|</span> <span class="hljs-number">15625.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, equal:[eq(test.t1.id, test.t2.id) eq(test.t3.id, test.t2.id)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_36(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_35 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_35 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─IndexJoin_28(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">12500.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_25, <span class="hljs-keyword">outer</span> key:test.t1.id, <span class="hljs-keyword">inner</span> key:test.t3.id, equal cond:eq(test.t1.id, test.t3.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_34(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_33 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_25(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_24 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_24 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t3 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t1.id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">---------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-number">9</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.01</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="使用-hint-导致错误-cant-find-a-proper-physical-plan-for-this-query" style="position:relative">使用 Hint 导致错误 <code>Can't find a proper physical plan for this query</code><a href="#%E4%BD%BF%E7%94%A8-hint-%E5%AF%BC%E8%87%B4%E9%94%99%E8%AF%AF-cant-find-a-proper-physical-plan-for-this-query" aria-label="使用 hint 导致错误 cant find a proper physical plan for this query permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p>在下面几种情况下，可能会出现 <code>Can't find a proper physical plan for this query</code> 错误：</p><ul><li>查询本身并不需要按顺序读取索引，即在不使用 Hint 的前提下，优化器在任何情况下都不会生成按顺序读取索引的计划。此时，如果指定了 <code>ORDER_INDEX</code> Hint，会出现此报错，此时应考虑移除对应的 <code>ORDER_INDEX</code> Hint。</li><li>查询使用了 <code>NO_JOIN</code> 相关的 Hint 排除了所有可能的 Join 方式。</li></ul><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1 (a <span class="hljs-type">INT</span>); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2 (a <span class="hljs-type">INT</span>); EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_HASH_JOIN(t1), NO_MERGE_JOIN(t1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a<span class="hljs-operator">=</span>t2.a; ERROR <span class="hljs-number">1815</span> (HY000): Internal : Can<span class="hljs-string">'t find a proper physical plan for this query </span></div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><ul><li>系统变量 <a href="/zh/tidb/stable/system-variables/#tidb_opt_enable_hash_join-从-v656v712-和-v740-版本开始引入"><code>tidb_opt_enable_hash_join</code></a> 设置为 <code>OFF</code>，而且其他 Join 方式也都被排除了。</li></ul><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1 (a <span class="hljs-type">INT</span>); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2 (a <span class="hljs-type">INT</span>); <span class="hljs-keyword">set</span> tidb_opt_enable_hash_join<span class="hljs-operator">=</span>off; EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ NO_MERGE_JOIN(t1) */</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1, t2 <span class="hljs-keyword">WHERE</span> t1.a<span class="hljs-operator">=</span>t2.a; ERROR <span class="hljs-number">1815</span> (HY000): Internal : Can<span class="hljs-string">'t find a proper physical plan for this query </span></div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><h3 id="set_var-写在子查询中不生效" style="position:relative"><code>SET_VAR</code> 写在子查询中不生效<a href="#set_var-%E5%86%99%E5%9C%A8%E5%AD%90%E6%9F%A5%E8%AF%A2%E4%B8%AD%E4%B8%8D%E7%94%9F%E6%95%88" aria-label="set_var 写在子查询中不生效 permalink" class="anchor after"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a></h3><p><code>SET_VAR</code> 用来设置当前语句的系统变量，不要写在子查询中。如果写在子查询中，由于子查询会被特殊处理，可能导致 <code>SET_VAR</code> 无法生效。</p><p>下面示例把 <code>SET_VAR</code> 写在了子查询中，所以没有生效。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">SELECT</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME, a <span class="hljs-keyword">FROM</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ SET_VAR(MAX_EXECUTION_TIME=123) */</span> <span class="hljs-number">1</span> <span class="hljs-keyword">as</span> a) t; <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+---+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME <span class="hljs-operator">|</span> a <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+---+</span> <span class="hljs-operator">|</span> <span class="hljs-number">0</span> <span class="hljs-operator">|</span> <span class="hljs-number">1</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+---+</span> <span class="hljs-number">1</span> <span class="hljs-type">row</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>下面示例没有把 SET_VAR 写在子查询中，所以可以生效。</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">SELECT</span> <span class="hljs-comment">/*+ SET_VAR(MAX_EXECUTION_TIME=123) */</span> @<span class="hljs-variable">@MAX</span>_EXECUTION_TIME, a <span class="hljs-keyword">FROM</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-number">1</span> <span class="hljs-keyword">as</span> a) t; <span class="hljs-operator">+</span><span class="hljs-comment">----------------------+---+</span>