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.MuiPaper-root{border-radius:6px;margin-top:8px;min-width:268px;color:rgb(55, 65, 81);box-shadow:rgb(255, 255, 255) 0px 0px 0px 0px,rgba(0, 0, 0, 0.05) 0px 0px 0px 1px,rgba(0, 0, 0, 0.1) 0px 10px 15px -3px,rgba(0, 0, 0, 0.05) 0px 4px 6px -2px;}.css-oe3wem .MuiPaper-root .MuiMenu-list{padding:4px 0;}.css-oe3wem .MuiPaper-root .MuiMenuItem-root .MuiSvgIcon-root{font-size:18px;color:rgba(0, 0, 0, 0.6);margin-right:12px;}.css-oe3wem .MuiPaper-root .MuiMenuItem-root:active{background-color:rgba(25, 118, 210, 0.08);}</style><style data-emotion="css 12mehxg">.css-12mehxg{padding:0;margin:0;list-style:none;outline:0;}</style><ul role="tree" id="left-nav-treeview" aria-multiselectable="false" class="MuiTreeView-root css-12mehxg" tabindex="0" aria-label="left navigation"><style data-emotion="css 1gesgke">.css-1gesgke .MuiTreeItem-content{color:#282a36;border-radius:4px;}.css-1gesgke .MuiTreeItem-content:hover{background-color:#f9f9f9;}.css-1gesgke .MuiTreeItem-content.Mui-selected,.css-1gesgke .MuiTreeItem-content.Mui-selected.Mui-focused{background-color:var(--tree-view-bg-color, #EAF6FB);color:var(--tree-view-color, #0A85C2);}.css-1gesgke .MuiTreeItem-content.Mui-selected svg.MuiTreeItem-ChevronRightIcon,.css-1gesgke .MuiTreeItem-content.Mui-selected.Mui-focused svg.MuiTreeItem-ChevronRightIcon{fill:var(--tree-view-color, #0A85C2);}.css-1gesgke .MuiTreeItem-content.Mui-focused{background-color:#f9f9f9;}.css-1gesgke .MuiTreeItem-content .MuiTreeItem-label{font-weight:inherit;color:inherit;}.css-1gesgke .MuiTreeItem-content .MuiTreeItem-iconContainer{;}.css-1gesgke .MuiTreeItem-group{margin-left:0;}.css-1gesgke .MuiTreeItem-group .MuiTreeItem-content{padding-left:16px;}</style><style data-emotion="css 1oy5rnm">.css-1oy5rnm{list-style:none;margin:0;padding:0;outline:0;}.css-1oy5rnm .MuiTreeItem-content{color:#282a36;border-radius:4px;}.css-1oy5rnm .MuiTreeItem-content:hover{background-color:#f9f9f9;}.css-1oy5rnm .MuiTreeItem-content.Mui-selected,.css-1oy5rnm .MuiTreeItem-content.Mui-selected.Mui-focused{background-color:var(--tree-view-bg-color, #EAF6FB);color:var(--tree-view-color, #0A85C2);}.css-1oy5rnm .MuiTreeItem-content.Mui-selected svg.MuiTreeItem-ChevronRightIcon,.css-1oy5rnm .MuiTreeItem-content.Mui-selected.Mui-focused svg.MuiTreeItem-ChevronRightIcon{fill:var(--tree-view-color, #0A85C2);}.css-1oy5rnm .MuiTreeItem-content.Mui-focused{background-color:#f9f9f9;}.css-1oy5rnm .MuiTreeItem-content .MuiTreeItem-label{font-weight:inherit;color:inherit;}.css-1oy5rnm .MuiTreeItem-content .MuiTreeItem-iconContainer{;}.css-1oy5rnm .MuiTreeItem-group{margin-left:0;}.css-1oy5rnm .MuiTreeItem-group .MuiTreeItem-content{padding-left:16px;}</style><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-0" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><style data-emotion="css khel7y">.css-khel7y{padding:0 8px;width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;cursor:pointer;-webkit-tap-highlight-color:transparent;}.css-khel7y:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-khel7y:hover{background-color:transparent;}}.css-khel7y.Mui-disabled{opacity:0.38;background-color:transparent;}.css-khel7y.Mui-focused{background-color:rgba(0, 0, 0, 0.12);}.css-khel7y.Mui-selected{background-color:rgba(25, 118, 210, 0.08);}.css-khel7y.Mui-selected:hover{background-color:rgba(25, 118, 210, 0.12);}@media (hover: none){.css-khel7y.Mui-selected:hover{background-color:rgba(25, 118, 210, 0.08);}}.css-khel7y.Mui-selected.Mui-focused{background-color:rgba(25, 118, 210, 0.2);}.css-khel7y .MuiTreeItem-iconContainer{margin-right:4px;width:15px;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-shrink:0;-ms-flex-negative:0;flex-shrink:0;-webkit-box-pack:center;-ms-flex-pack:center;-webkit-justify-content:center;justify-content:center;}.css-khel7y .MuiTreeItem-iconContainer svg{font-size:18px;}.css-khel7y .MuiTreeItem-label{width:100%;min-width:0;padding-left:4px;position:relative;font-size:1rem;line-height:1.5rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:#666666;}</style><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><style data-emotion="css znl8a3">.css-znl8a3{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:row;-ms-flex-direction:row;flex-direction:row;min-height:1.75rem;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;padding-left:0rem;padding-top:0.25rem;padding-bottom:0.25rem;}</style><div class="css-znl8a3"><style data-emotion="css tx8jsd">.css-tx8jsd{margin:0;font-size:1rem;line-height:1.5rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:#666666;}</style><a class="MuiTypography-root MuiTypography-body1 css-tx8jsd" target="_blank" href="https://docs.pingcap.com/zh" style="text-decoration:none;width:100%;color:inherit"><style data-emotion="css 8atqhb">.css-8atqhb{width:100%;}</style><div class="MuiBox-root css-8atqhb"><style data-emotion="css 1hwyipb">.css-1hwyipb{margin:0;font-size:1rem;line-height:1.5rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:#666666;color:inherit;font-size:0.875rem;line-height:1.25rem;}</style><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">文档中心</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-1" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">关于 TiDB</div></div><style data-emotion="css xjq79n">.css-xjq79n{-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;width:1em;height:1em;display:inline-block;fill:currentColor;-webkit-flex-shrink:0;-ms-flex-negative:0;flex-shrink:0;-webkit-transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;font-size:1.5rem;fill:#999999;height:1rem;width:1rem;margin-left:auto;-webkit-transform:none;-moz-transform:none;-ms-transform:none;transform:none;}</style><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div><style data-emotion="css 1jozaee">.css-1jozaee{margin:0;padding:0;margin-left:17px;}</style></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-2" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">快速上手</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-3" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">应用开发</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-4" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">部署标准集群</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-5" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">数据迁移</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-6" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">数据集成</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-7" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">运维操作</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-8" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">监控与告警</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-9" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">故障诊断</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="true" id="left-nav-treeview-0-10" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content Mui-expanded" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">性能调优</div></div><style data-emotion="css 171pefp">.css-171pefp{-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;width:1em;height:1em;display:inline-block;fill:currentColor;-webkit-flex-shrink:0;-ms-flex-negative:0;flex-shrink:0;-webkit-transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;font-size:1.5rem;fill:#999999;height:1rem;width:1rem;margin-left:auto;-webkit-transform:rotate(90deg);-moz-transform:rotate(90deg);-ms-transform:rotate(90deg);transform:rotate(90deg);}</style><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-171pefp" 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></div></div><style data-emotion="css 1xxsnna">.css-1xxsnna{height:auto;overflow:visible;-webkit-transition:height 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:height 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;margin:0;padding:0;margin-left:17px;}</style><ul class="MuiCollapse-root MuiCollapse-vertical MuiTreeItem-group MuiCollapse-entered css-1xxsnna" style="min-height:0px" role="group"><div class="MuiCollapse-wrapper MuiCollapse-vertical css-hboir5"><div class="MuiCollapse-wrapperInner MuiCollapse-vertical css-8atqhb"><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-10-0" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><style data-emotion="css kb2j4b">.css-kb2j4b{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:row;-ms-flex-direction:row;flex-direction:row;min-height:1.75rem;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;padding-left:0.5rem;padding-top:0.25rem;padding-bottom:0.25rem;}</style><div class="css-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">优化手册</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-10-1" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">配置调优</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-xjq79n" 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></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="true" id="left-nav-treeview-0-10-2" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content Mui-expanded" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">SQL 性能调优</div></div><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium MuiTreeItem-ChevronRightIcon css-171pefp" 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></div></div><ul class="MuiCollapse-root MuiCollapse-vertical MuiTreeItem-group MuiCollapse-entered css-1xxsnna" style="min-height:0px" role="group"><div class="MuiCollapse-wrapper MuiCollapse-vertical css-hboir5"><div class="MuiCollapse-wrapperInner MuiCollapse-vertical css-8atqhb"><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-0" tabindex="-1" style="margin-top:0.1875rem;margin-bottom:0.1875rem"><div class="css-khel7y MuiTreeItem-content" style="width:inherit"><div class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><style data-emotion="css 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(min-width:0px){.css-90fn5w{-webkit-flex-direction:column-reverse;-ms-flex-direction:column-reverse;flex-direction:column-reverse;}}@media (min-width:600px){.css-90fn5w{-webkit-flex-direction:row;-ms-flex-direction:row;flex-direction:row;}}</style><div class="css-90fn5w"><style data-emotion="css vofosd">.css-vofosd{padding:1.5rem 0;}@media (min-width:0px){.css-vofosd{width:100%;}}@media (min-width:600px){.css-vofosd{width:calc(100% - 17.5rem);}}</style><div class="MuiBox-root css-vofosd"><style data-emotion="css 1ekb41w">.css-1ekb41w{width:100%;margin-left:auto;box-sizing:border-box;margin-right:auto;display:block;padding-left:16px;padding-right:16px;}@media (min-width:600px){.css-1ekb41w{padding-left:24px;padding-right:24px;}}@media (min-width:1536px){.css-1ekb41w{max-width:1536px;}}</style><div class="MuiContainer-root MuiContainer-maxWidthXl doc-content css-1ekb41w"><div class="markdown-body MuiBox-root css-0"><h1 id="prepare-语句执行计划缓存" style="position:relative"><a href="#prepare-%E8%AF%AD%E5%8F%A5%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92%E7%BC%93%E5%AD%98" aria-label="prepare 语句执行计划缓存 permalink" class="anchor before"><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>Prepare 语句执行计划缓存</h1><p>TiDB 支持对 <code>Prepare</code>/<code>Execute</code> 请求的执行计划缓存。其中包括以下两种形式的预处理语句：</p><ul><li>使用 <code>COM_STMT_PREPARE</code> 和 <code>COM_STMT_EXECUTE</code> 的协议功能；</li><li>执行 <code>Prepare</code>/<code>Execute</code> SQL 语句查询；</li></ul><p>TiDB 优化器对这两类查询的处理是一样的：<code>Prepare</code> 时将参数化的 SQL 查询解析成 AST（抽象语法树），每次 <code>Execute</code> 时根据保存的 AST 和具体的参数值生成执行计划。</p><p>当开启执行计划缓存后，每条 <code>Prepare</code> 语句的第一次 <code>Execute</code> 会检查当前查询是否可以使用执行计划缓存，如果可以则将生成的执行计划放进一个由 LRU 链表构成的缓存中；在后续的 <code>Execute</code> 中，会先从缓存中获取执行计划，并检查是否可用，如果获取和检查成功则跳过生成执行计划这一步，否则重新生成执行计划并放入缓存中。</p><p>对于某些非 <code>PREPARE</code> 语句，TiDB 可以像 <code>Prepare</code>/<code>Execute</code> 语句一样支持执行计划缓存，详情请参考<a href="/zh/tidb/stable/sql-non-prepared-plan-cache">非 Prepare 语句执行计划缓存</a>。</p><p>在当前版本中，当 <code>Prepare</code> 语句符合以下条件任何一条，查询或者计划不会被缓存：</p><ul><li><code>SELECT</code>、<code>UPDATE</code>、<code>INSERT</code>、<code>DELETE</code>、<code>Union</code>、<code>Intersect</code>、<code>Except</code> 以外的 SQL 语句；</li><li>访问分区表、临时表或访问表中包含生成列的查询；</li><li>查询中包含非关联子查询，例如 <code>SELECT * FROM t1 WHERE t1.a &gt; (SELECT 1 FROM t2 WHERE t2.b &lt; 1)</code>；</li><li>执行计划中带有 <code>PhysicalApply</code> 算子的关联子查询，例如 <code>SELECT * FROM t1 WHERE t1.a &gt; (SELECT a FROM t2 WHERE t1.b &gt; t2.b)</code>；</li><li>包含 <code>ignore_plan_cache</code> 或 <code>set_var</code> 这两个 Hint 的查询，例如 <code>SELECT /*+ ignore_plan_cache() */ * FROM t</code> 或 <code>SELECT /*+ set_var(max_execution_time=1) */ * FROM t</code>；</li><li>包含除 <code>?</code> 外其他变量（即系统变量或用户自定义变量）的查询，例如 <code>select * from t where a&gt;? and b&gt;@x</code>；</li><li>查询包含无法被缓存函数。目前不能被缓存的函数有：<code>database()</code>、<code>current_user</code>、<code>current_role</code>、<code>user</code>、<code>connection_id</code>、<code>last_insert_id</code>、<code>row_count</code>、<code>version</code>、<code>like</code>；</li><li><code>LIMIT</code> 后面带有变量（例如 <code>LIMIT ?</code> 或 <code>LIMIT 10, ?</code>）且变量值大于 10000 的执行计划不缓存；</li><li><code>?</code> 直接在 <code>Order By</code> 后的查询，如 <code>Order By ?</code>，此时 <code>?</code> 表示根据 <code>Order By</code> 后第几列排序，排序列不同的查询使用同一个计划可能导致错误结果，故不缓存；如果是普通表达式，如 <code>Order By a+?</code> 则会缓存；</li><li><code>?</code> 紧跟在 <code>Group by</code> 后的查询，如 <code>Group By ?</code>，此时 <code>?</code> 表示根据 <code>Group By</code> 后第几列聚合，聚合列不同的查询使用同一个计划可能导致错误结果，故不缓存；如果是普通表达式，如 <code>Group By a+?</code> 则会缓存；</li><li><code>?</code> 出现在窗口函数 <code>Window Frame</code> 定义中的查询，如 <code>(partition by year order by sale rows ? preceding)</code>；如果 <code>?</code> 出现在窗口函数的其他位置，则会缓存；</li><li>用参数进行 <code>int</code> 和 <code>string</code> 比较的查询，如 <code>c_int &gt;= ?</code> 或者 <code>c_int in (?, ?)</code>等，其中 <code>?</code> 为字符串类型，如 <code>set @x='123'</code>；此时为了保证结果和 MySQL 兼容性，需要每次对参数进行调整，故不会缓存；</li><li>会访问 <code>TiFlash</code> 的计划不会被缓存；</li><li>大部分情况下计划中含有 <code>TableDual</code> 的计划将将不会被缓存，除非当前执行的 <code>Prepare</code> 语句不含参数，则对应的 <code>TableDual</code> 计划可以被缓存。</li><li>访问 TiDB 系统视图的查询，如 <code>information_schema.columns</code>。不建议使用 <code>Prepare</code>/<code>Execute</code> 语句访问系统视图。</li></ul><p>TiDB 对 <code>?</code> 的个数有限制，如果超过了 65535 个，则会报错 <code>Prepared statement contains too many placeholders</code>。</p><p>LRU 链表是设计成 session 级别的缓存，因为 <code>Prepare</code>/<code>Execute</code> 不能跨 session 执行。LRU 链表的每个元素是一个 key-value 对，value 是执行计划，key 由如下几部分组成：</p><ul><li>执行 <code>Execute</code> 时所在数据库的名字；</li><li><code>Prepare</code> 语句的标识符，即紧跟在 <code>PREPARE</code> 关键字后的名字；</li><li>当前的 schema 版本，每条执行成功的 DDL 语句会修改 schema 版本；</li><li>执行 <code>Execute</code> 时的 SQL Mode；</li><li>当前设置的时区，即系统变量 <code>time_zone</code> 的值；</li><li>系统变量 <code>sql_select_limit</code> 的值；</li></ul><p>key 中任何一项变动（如切换数据库、重命名 <code>Prepare</code> 语句、执行 DDL、修改 SQL Mode/<code>time_zone</code> 的值）、或 LRU 淘汰机制触发都会导致 <code>Execute</code> 时无法命中执行计划缓存。</p><p>成功从缓存中获取到执行计划后，TiDB 会先检查执行计划是否依然合法，如果当前 <code>Execute</code> 在显式事务里执行，并且引用的表在事务前序语句中被修改，而缓存的执行计划对该表访问不包含 <code>UnionScan</code> 算子，则它不能被执行。</p><p>在通过合法性检测后，会根据当前最新参数值，对执行计划的扫描范围做相应调整，再用它执行获取数据。</p><p>关于执行计划缓存和查询性能有几点值得注意：</p><ul><li>不管计划是否已经被缓存，都会受到 SQL Binding 的影响。对于没有被缓存的计划，即在第一次执行 <code>Execute</code> 时，会受到已有 SQL Binding 的影响；而对于已经缓存的计划，如果有新的 SQL Binding 被创建产生，则原有已经被缓存的计划会失效。</li><li>已经被缓存的计划不会受到统计信息更新、优化规则和表达式下推黑名单更新的影响，仍然会使用已经保存在缓存中的计划。</li><li>重启 TiDB 实例时（如不停机滚动升级 TiDB 集群），<code>Prepare</code> 信息会丢失，此时执行 <code>execute stmt ...</code> 可能会遇到 <code>Prepared Statement not found</code> 的错误，此时需要再执行一次 <code>prepare stmt ...</code>。</li><li>考虑到不同 <code>Execute</code> 的参数会不同，执行计划缓存为了保证适配性会禁止一些和具体参数值密切相关的激进查询优化手段，导致对特定的一些参数值，查询计划可能不是最优。比如查询的过滤条件为 <code>where a &gt; ? and a &lt; ?</code>，第一次 <code>Execute</code> 时参数分别为 2 和 1，考虑到这两个参数下次执行时可能会是 1 和 2，优化器不会生成对当前参数最优的 <code>TableDual</code> 执行计划。</li><li>如果不考虑缓存失效和淘汰，一份执行计划缓存会对应各种不同的参数取值，理论上也会导致某些取值下执行计划非最优。比如查询过滤条件为 <code>where a &lt; ?</code>，假如第一次执行 <code>Execute</code> 时用的参数值为 1，此时优化器生成最优的 <code>IndexScan</code> 执行计划放入缓存，在后续执行 <code>Exeucte</code> 时参数变为 10000，此时 <code>TableScan</code> 可能才是更优执行计划，但由于执行计划缓存，执行时还是会使用先前生成的 <code>IndexScan</code>。因此执行计划缓存更适用于查询较为简单（查询编译耗时占比较高）且执行计划较为固定的业务场景。</li></ul><p>自 v6.1.0 起，执行计划缓存功能默认打开，可以通过变量 <a href="/zh/tidb/stable/system-variables#tidb_enable_prepared_plan_cache-从-v610-版本开始引入"><code>tidb_enable_prepared_plan_cache</code></a> 启用或关闭这项功能。</p><style data-emotion="css 1weqspv">.css-1weqspv{padding-top:0.5rem;padding-bottom:0.5rem;}.css-1weqspv .MuiAlert-message p{margin-bottom:0px;margin-top:0px;}</style><div class="MuiBox-root css-1weqspv"><style data-emotion="css 17zlexa">.css-17zlexa{font-size:0.875rem;line-height:1.25rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:rgb(1, 67, 97);background-color:rgb(229, 246, 253);display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;padding:6px 16px;}.css-17zlexa .MuiAlert-icon{color:#03a9f4;}</style><style data-emotion="css 152gid5">.css-152gid5{background-color:#fff;color:rgba(0, 0, 0, 0.87);-webkit-transition:box-shadow 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:box-shadow 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;border-radius:4px;box-shadow:none;font-size:0.875rem;line-height:1.25rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:rgb(1, 67, 97);background-color:rgb(229, 246, 253);display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;padding:6px 16px;}.css-152gid5 .MuiAlert-icon{color:#03a9f4;}</style><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-152gid5" role="alert"><style data-emotion="css 1l54tgj">.css-1l54tgj{margin-right:12px;padding:7px 0;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;font-size:22px;opacity:0.9;}</style><div class="MuiAlert-icon css-1l54tgj"><style data-emotion="css 1cw4hi4">.css-1cw4hi4{-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;width:1em;height:1em;display:inline-block;fill:currentColor;-webkit-flex-shrink:0;-ms-flex-negative:0;flex-shrink:0;-webkit-transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;font-size:inherit;}</style><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><style data-emotion="css 1xsto0d">.css-1xsto0d{padding:8px 0;min-width:0;overflow:auto;}</style><div class="MuiAlert-message css-1xsto0d"><style data-emotion="css 1h2y6o4">.css-1h2y6o4{font-weight:500;margin-top:-2px;}</style><style data-emotion="css 17ioyuk">.css-17ioyuk{margin:0;font-size:1rem;line-height:1.5rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:#666666;margin-bottom:0.35em;font-weight:500;margin-top:-2px;}</style><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-17ioyuk">注意</div><p>执行计划缓存功能仅针对 <code>Prepare</code>/<code>Execute</code> 请求，对普通查询无效。</p></div></div></div><p>在开启了执行计划缓存功能后，可以通过 SESSION 级别的系统变量 <a href="/zh/tidb/stable/system-variables#last_plan_from_cache-从-v40-版本开始引入"><code>last_plan_from_cache</code></a> 查看上一条 <code>Execute</code> 语句是否使用了缓存的执行计划，例如：</p><pre><code class="hljs language-sql"><style data-emotion="css 1qhimia">.css-1qhimia{margin-right:3rem;overflow:auto;}</style><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> t(a <span class="hljs-type">int</span>); Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'select * from t where a = ?'</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">set</span> <span class="hljs-variable">@a</span> <span class="hljs-operator">=</span> <span class="hljs-number">1</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) <span class="hljs-comment">-- 第一次 execute 生成执行计划放入缓存</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> <span class="hljs-variable">@a</span>; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <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) <span class="hljs-comment">-- 第二次 execute 命中缓存</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> <span class="hljs-variable">@a</span>; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</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><style data-emotion="css ux9q70">.css-ux9q70{text-align:center;-webkit-flex:0 0 auto;-ms-flex:0 0 auto;flex:0 0 auto;font-size:1.5rem;padding:8px;border-radius:50%;overflow:visible;color:rgba(0, 0, 0, 0.54);-webkit-transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;padding:5px;font-size:1.125rem;top:0.625rem;right:2.4rem;background:transparent;border:unset;}.css-ux9q70:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-ux9q70:hover{background-color:transparent;}}.css-ux9q70.Mui-disabled{background-color:transparent;color:rgba(0, 0, 0, 0.26);}</style><style data-emotion="css 1rmx1rm">.css-1rmx1rm{display:-webkit-inline-box;display:-webkit-inline-flex;display:-ms-inline-flexbox;display:inline-flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-ms-flex-pack:center;-webkit-justify-content:center;justify-content:center;position:relative;box-sizing:border-box;-webkit-tap-highlight-color:transparent;background-color:transparent;outline:0;border:0;margin:0;border-radius:0;padding:0;cursor:pointer;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;vertical-align:middle;-moz-appearance:none;-webkit-appearance:none;-webkit-text-decoration:none;text-decoration:none;color:inherit;text-align:center;-webkit-flex:0 0 auto;-ms-flex:0 0 auto;flex:0 0 auto;font-size:1.5rem;padding:8px;border-radius:50%;overflow:visible;color:rgba(0, 0, 0, 0.54);-webkit-transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;padding:5px;font-size:1.125rem;top:0.625rem;right:2.4rem;background:transparent;border:unset;}.css-1rmx1rm::-moz-focus-inner{border-style:none;}.css-1rmx1rm.Mui-disabled{pointer-events:none;cursor:default;}@media print{.css-1rmx1rm{-webkit-print-color-adjust:exact;color-adjust:exact;}}.css-1rmx1rm:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-1rmx1rm:hover{background-color:transparent;}}.css-1rmx1rm.Mui-disabled{background-color:transparent;color:rgba(0, 0, 0, 0.26);}</style><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><style data-emotion="css 1e2dcm1">.css-1e2dcm1{z-index:1500;pointer-events:none;}</style><style data-emotion="css okvapm">.css-okvapm{z-index:1500;pointer-events:none;}</style><style data-emotion="css rnn29l">.css-rnn29l{text-align:center;-webkit-flex:0 0 auto;-ms-flex:0 0 auto;flex:0 0 auto;font-size:1.5rem;padding:8px;border-radius:50%;overflow:visible;color:rgba(0, 0, 0, 0.54);-webkit-transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;padding:5px;font-size:1.125rem;top:0.625rem;right:0.625rem;background:transparent;border:unset;}.css-rnn29l:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-rnn29l:hover{background-color:transparent;}}.css-rnn29l.Mui-disabled{background-color:transparent;color:rgba(0, 0, 0, 0.26);}</style><style data-emotion="css fxo539">.css-fxo539{display:-webkit-inline-box;display:-webkit-inline-flex;display:-ms-inline-flexbox;display:inline-flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-ms-flex-pack:center;-webkit-justify-content:center;justify-content:center;position:relative;box-sizing:border-box;-webkit-tap-highlight-color:transparent;background-color:transparent;outline:0;border:0;margin:0;border-radius:0;padding:0;cursor:pointer;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;vertical-align:middle;-moz-appearance:none;-webkit-appearance:none;-webkit-text-decoration:none;text-decoration:none;color:inherit;text-align:center;-webkit-flex:0 0 auto;-ms-flex:0 0 auto;flex:0 0 auto;font-size:1.5rem;padding:8px;border-radius:50%;overflow:visible;color:rgba(0, 0, 0, 0.54);-webkit-transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:background-color 150ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;padding:5px;font-size:1.125rem;top:0.625rem;right:0.625rem;background:transparent;border:unset;}.css-fxo539::-moz-focus-inner{border-style:none;}.css-fxo539.Mui-disabled{pointer-events:none;cursor:default;}@media print{.css-fxo539{-webkit-print-color-adjust:exact;color-adjust:exact;}}.css-fxo539:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-fxo539:hover{background-color:transparent;}}.css-fxo539.Mui-disabled{background-color:transparent;color:rgba(0, 0, 0, 0.26);}</style><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>如果发现某一组 <code>Prepare</code>/<code>Execute</code> 由于执行计划缓存导致了非预期行为，可以通过 SQL Hint <code>ignore_plan_cache()</code> 让该组语句不使用缓存。还是用上述的 <code>stmt</code> 为例：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'select /*+ ignore_plan_cache() */ * from t where a = ?'</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">set</span> <span class="hljs-variable">@a</span> <span class="hljs-operator">=</span> <span class="hljs-number">1</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> <span class="hljs-variable">@a</span>; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <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) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> <span class="hljs-variable">@a</span>; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <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><h2 id="诊断-prepared-plan-cache" style="position:relative"><a href="#%E8%AF%8A%E6%96%AD-prepared-plan-cache" aria-label="诊断 prepared plan cache permalink" class="anchor before"><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>诊断 Prepared Plan Cache</h2><p>对于无法进行缓存的查询或计划，可通过 <code>SHOW WARNINGS</code> 语句查看查询或计划是否被缓存。如果未被缓存，则可在结果中查看无法被缓存的原因。示例如下：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">PREPARE</span> st <span class="hljs-keyword">FROM</span> <span class="hljs-string">'SELECT * FROM t WHERE a &gt; (SELECT MAX(a) FROM t)'</span>; <span class="hljs-comment">-- 该查询包含子查询，因此无法被缓存</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected, <span class="hljs-number">1</span> warning (<span class="hljs-number">0.01</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">SHOW</span> WARNINGS; <span class="hljs-comment">-- 查看查询计划无法被缓存的原因</span> <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">1105</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">skip</span> plan<span class="hljs-operator">-</span>cache: sub<span class="hljs-operator">-</span>queries <span class="hljs-keyword">are</span> un<span class="hljs-operator">-</span>cacheable <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) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">PREPARE</span> st <span class="hljs-keyword">FROM</span> <span class="hljs-string">'SELECT * FROM t WHERE a&lt;?'</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">SET</span> <span class="hljs-variable">@a</span><span class="hljs-operator">=</span><span class="hljs-string">'1'</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">EXECUTE</span> st <span class="hljs-keyword">USING</span> <span class="hljs-variable">@a</span>; <span class="hljs-comment">-- 该优化中进行了非 INT 类型到 INT 类型的转换，产生的执行计划可能随着参数变化而存在风险，因此 TiDB 不缓存该计划</span> <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span>, <span class="hljs-number">1</span> warning (<span class="hljs-number">0.01</span> sec) mysql<span class="hljs-operator">&gt;</span> <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">1105</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">skip</span> plan<span class="hljs-operator">-</span>cache: <span class="hljs-string">'1'</span> may be converted <span class="hljs-keyword">to</span> <span class="hljs-type">INT</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><h2 id="prepared-plan-cache-的内存管理" style="position:relative"><a href="#prepared-plan-cache-%E7%9A%84%E5%86%85%E5%AD%98%E7%AE%A1%E7%90%86" aria-label="prepared plan cache 的内存管理 permalink" class="anchor before"><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>Prepared Plan Cache 的内存管理</h2><p>使用 Prepared Plan Cache 会有一定的内存开销，可以通过 Grafana 中的 <a href="/zh/tidb/stable/grafana-tidb-dashboard"><code>Plan Cache Memory Usage</code> 监控</a>查看每台 TiDB 实例上所有 <code>SESSION</code> 所缓存的计划占用的总内存。</p><div class="MuiBox-root css-1weqspv"><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-152gid5" 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-17ioyuk">注意</div><p>考虑到 Golang 的内存回收机制以及部分未统计的内存结构，Grafana 中显示的内存与实际的堆内存使用量并不相等。经过实验验证存在约 ±20% 的误差。</p></div></div></div><p>对于每台 TiDB 实例上所缓存的执行计划总数量，可以通过 Grafana 中的 <a href="/zh/tidb/stable/grafana-tidb-dashboard"><code>Plan Cache Plan Num</code> 监控</a>查看。</p><p>Grafana 中 <code>Plan Cache Memory Usage</code> 和 <code>Plan Cache Plan Num</code> 监控如下图所示：</p><p><img src="https://download.pingcap.com/images/docs-cn/planCache-memoryUsage-planNum-panels.png" alt="grafana_panels"/></p><p>从 v7.1.0 开始，你可以通过变量 <a href="/zh/tidb/stable/system-variables#tidb_session_plan_cache_size-从-v710-版本开始引入"><code>tidb_session_plan_cache_size</code></a> 来设置每个 <code>SESSION</code> 最多缓存的计划数量。针对不同的环境，推荐的设置如下，你可以结合监控进行调整：</p><ul><li>TiDB Server 实例内存阈值 &lt;= 64 GiB 时，<code>tidb_session_plan_cache_size = 50</code></li><li>TiDB Server 实例内存阈值 &gt; 64 GiB 时，<code>tidb_session_plan_cache_size = 100</code></li></ul><p>从 v7.1.0 开始，你可以通过变量 <a href="/zh/tidb/stable/system-variables#tidb_plan_cache_max_plan_size-从-v710-版本开始引入"><code>tidb_plan_cache_max_plan_size</code></a> 来设置可以缓存的计划的最大大小，默认为 2 MB。超过该值的执行计划将不会被缓存到 Plan Cache 中。</p><p>当 TiDB Server 的内存余量小于一定阈值时，会触发 Plan Cache 的内存保护机制，此时会对一些缓存的计划进行逐出。</p><p>目前该阈值由变量 <code>tidb_prepared_plan_cache_memory_guard_ratio</code> 控制，默认为 0.1，即 10%，也就是当剩余内存不足 10%（使用内存超过 90%）时，会触发此机制。</p><p>由于内存限制，Plan Cache 可能出现 Cache Miss 的情况，可以通过 Grafana 中的 <a href="/zh/tidb/stable/grafana-tidb-dashboard"><code>Plan Cache Miss OPS</code> 监控</a>查看。</p><h2 id="手动清空计划缓存" style="position:relative"><a href="#%E6%89%8B%E5%8A%A8%E6%B8%85%E7%A9%BA%E8%AE%A1%E5%88%92%E7%BC%93%E5%AD%98" aria-label="手动清空计划缓存 permalink" class="anchor before"><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>手动清空计划缓存</h2><p>通过执行 <code>ADMIN FLUSH [SESSION | INSTANCE] PLAN_CACHE</code> 语句，你可以手动清空计划缓存。</p><p>该语句中的作用域 <code>[SESSION | INSTANCE]</code> 用于指定需要清空的缓存级别，可以为 <code>SESSION</code> 或 <code>INSTANCE</code>。如果不指定作用域，该语句默认清空 <code>SESSION</code> 级别的缓存。</p><p>下面是一个清空计划缓存的例子：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> t (a <span class="hljs-type">int</span>); Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'select * from t'</span>; Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-comment">-- 选择计划缓存</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</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) MySQL [test]<span class="hljs-operator">&gt;</span> admin flush session plan_cache; <span class="hljs-comment">-- 清空当前 session 的计划缓存</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt; <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-comment">-- 由于缓存被清空，此时无法再次选中</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span> <span class="hljs-operator">|</span> @<span class="hljs-variable">@last</span>_plan_from_cache <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><p>TiDB 暂不支持清空 <code>GLOBAL</code> 级别的计划缓存，即不支持一次性清空整个集群的计划缓存，使用时会报错：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> admin flush <span class="hljs-keyword">global</span> plan_cache; ERROR <span class="hljs-number">1105</span> (HY000): Do <span class="hljs-keyword">not</span> support the <span class="hljs-string">'admin flush global scope.'</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><h2 id="忽略-com_stmt_close-指令和-deallocate-prepare-语句" style="position:relative"><a href="#%E5%BF%BD%E7%95%A5-com_stmt_close-%E6%8C%87%E4%BB%A4%E5%92%8C-deallocate-prepare-%E8%AF%AD%E5%8F%A5" aria-label="忽略 com_stmt_close 指令和 deallocate prepare 语句 permalink" class="anchor before"><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>忽略 <code>COM_STMT_CLOSE</code> 指令和 <code>DEALLOCATE PREPARE</code> 语句</h2><p>为了减少每次执行 SQL 语句的语法分析，Prepared Statement 推荐的使用方式是，prepare 一次，然后 execute 多次，最后 deallocate prepare。例如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'...'</span>; <span class="hljs-comment">-- prepare 一次</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> ...; <span class="hljs-comment">-- execute 一次</span> MySQL [test]<span class="hljs-operator">&gt;</span> ... MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> ...; <span class="hljs-comment">-- execute 多次</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">deallocate</span> <span class="hljs-keyword">prepare</span> stmt; <span class="hljs-comment">-- 使用完成后释放</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>如果你习惯于在每次 execute 后都立即执行 deallocate prepare，如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'...'</span>; <span class="hljs-comment">-- 第一次 prepare</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> ...; MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">deallocate</span> <span class="hljs-keyword">prepare</span> stmt; <span class="hljs-comment">-- 一次使用后立即释放</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'...'</span>; <span class="hljs-comment">-- 第二次 prepare</span> MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt <span class="hljs-keyword">using</span> ...; MySQL [test]<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">deallocate</span> <span class="hljs-keyword">prepare</span> stmt; <span class="hljs-comment">-- 再次释放</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>这样的使用方式会让第一次执行得到的计划被立即清理，不能在第二次被复用。</p><p>为了兼容这样的使用方式，从 v6.0 起，TiDB 支持 <a href="/zh/tidb/stable/system-variables#tidb_ignore_prepared_cache_close_stmt-从-v600-版本开始引入"><code>tidb_ignore_prepared_cache_close_stmt</code></a> 变量。打开该变量后，TiDB 会忽略关闭 Prepare Statement 的信号，解决上述问题，如：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">set</span> @<span class="hljs-variable">@tidb</span>_ignore_prepared_cache_close_stmt<span class="hljs-operator">=</span><span class="hljs-number">1</span>; <span class="hljs-comment">-- 打开开关</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'select * from t'</span>; <span class="hljs-comment">-- 第一次 prepare</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt; <span class="hljs-comment">-- 第一次 execute</span> <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">deallocate</span> <span class="hljs-keyword">prepare</span> stmt; <span class="hljs-comment">-- 第一次 execute 后立即释放</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">prepare</span> stmt <span class="hljs-keyword">from</span> <span class="hljs-string">'select * from t'</span>; <span class="hljs-comment">-- 第二次 prepare</span> Query OK, <span class="hljs-number">0</span> <span class="hljs-keyword">rows</span> affected (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">execute</span> stmt; <span class="hljs-comment">-- 第二次 execute</span> <span class="hljs-keyword">Empty</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) mysql<span class="hljs-operator">&gt;</span> <span class="hljs-keyword">select</span> @<span class="hljs-variable">@last</span>_plan_from_cache; <span class="hljs-comment">-- 因为开关打开，第二次依旧能复用上一次的计划</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------+</span>