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.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">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-0-4" 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">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-0-5" 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">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-0-6" 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">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-0-7" 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">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="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">性能调优</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="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">教程</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" 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">TiDB 工具</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-0-11" 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">参考指南</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-0-11-0" 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">架构</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-0-11-1" 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">存储引擎 TiKV</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-0-11-2" 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">存储引擎 TiFlash</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-0-11-2-0" 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 snygwd">.css-snygwd{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:1rem;padding-top:0.25rem;padding-bottom:0.25rem;}</style><div class="css-snygwd"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/v5.0/tiflash-overview"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">TiFlash 简介</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-11-2-1" 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-snygwd"><a aria-current="page" style="text-decoration:none;width:100%;color:inherit" class="" href="/zh/tidb/v5.0/use-tiflash"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">使用 TiFlash</div></div></a></div></div></div></li></div></div></ul></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-11-3" 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-kb2j4b"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/v5.0/system-variables"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">系统变量</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-11-4" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">配置文件参数</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-0-11-5" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">CLI</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-0-11-6" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">命令行参数</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-0-11-7" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">监控指标</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-0-11-8" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">安全加固</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-0-11-9" 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-kb2j4b"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">权限</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-0-11-10" 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">SQL</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-0-11-11" 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">UI</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" id="left-nav-treeview-0-11-12" tabindex="-1" 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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" id="left-nav-treeview-0-14" 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"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/v5.0/glossary"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">术语表</div></div></a></div></div></div></li></ul></div></aside><style data-emotion="css sb2sd0">.css-sb2sd0{width:100%;}@media (min-width:1200px){.css-sb2sd0{max-width:calc(100% - 18.75rem);}}</style><main class="MuiBox-root css-sb2sd0"><style data-emotion="css 1rnnmjb">.css-1rnnmjb{max-width:1340px;width:100%;margin-left:auto;margin-right:auto;display:block;box-sizing:border-box;}</style><div class="MuiBox-root 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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">重要</div><p class="MuiTypography-root MuiTypography-body1 css-tx8jsd">你正在查看 TiDB 数据库的较旧版本 (TiDB v5.0) 的文档。如无特殊需求，建议使用 TiDB 数据库的<a hreflang="zh" href="/zh/tidb/stable">最新 LTS 版本</a>。</p></div></div></div><h1 id="使用-tiflash" style="position:relative"><a href="#%E4%BD%BF%E7%94%A8-tiflash" aria-label="使用 tiflash 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>使用 TiFlash</h1><p>TiFlash 部署完成后并不会自动同步数据，而需要手动指定需要同步的表。</p><p>用户可以使用 TiDB 或者 TiSpark 读取 TiFlash，TiDB 适合用于中等规模的 OLAP 计算，而 TiSpark 适合大规模的 OLAP 计算，用户可以根据自己的场景和使用习惯自行选择。具体参见：</p><ul><li><a href="#%E4%BD%BF%E7%94%A8-tidb-%E8%AF%BB%E5%8F%96-tiflash">使用 TiDB 读取 TiFlash</a></li><li><a href="#%E4%BD%BF%E7%94%A8-tispark-%E8%AF%BB%E5%8F%96-tiflash">使用 TiSpark 读取 TiFlash</a></li></ul><h2 id="按表构建-tiflash-副本" style="position:relative"><a href="#%E6%8C%89%E8%A1%A8%E6%9E%84%E5%BB%BA-tiflash-%E5%89%AF%E6%9C%AC" aria-label="按表构建 tiflash 副本 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>按表构建 TiFlash 副本</h2><p>TiFlash 接入 TiKV 集群后，默认不会开始同步数据。可通过 MySQL 客户端向 TiDB 发送 DDL 命令来为特定的表建立 TiFlash 副本：</p><pre><code class="language-sql">ALTER TABLE table_name SET TIFLASH REPLICA count </code></pre><p>该命令的参数说明如下：</p><ul><li>count 表示副本数，0 表示删除。</li></ul><p>对于相同表的多次 DDL 命令，仅保证最后一次能生效。例如下面给出的操作 <code>tpch50</code> 表的两条 DDL 命令中，只有第二条删除副本的命令能生效：</p><p>为表建立 2 个副本：</p><pre><code class="language-sql">ALTER TABLE `tpch50`.`lineitem` SET TIFLASH REPLICA 2 </code></pre><p>删除副本：</p><pre><code class="language-sql">ALTER TABLE `tpch50`.`lineitem` SET TIFLASH REPLICA 0 </code></pre><p>注意事项：</p><ul><li><p>假设有一张表 t 已经通过上述的 DDL 语句同步到 TiFlash，则通过以下语句创建的表也会自动同步到 TiFlash：</p><pre><code class="language-sql">CREATE TABLE table_name like t </code></pre></li><li><p>如果集群版本 \&lt; v4.0.6，若先对表创建 TiFlash 副本，再使用 TiDB Lightning 导入数据，会导致数据导入失败。需要在使用 TiDB Lightning 成功导入数据至表后，再对相应的表创建 TiFlash 副本。</p></li><li><p>如果集群版本以及 TiDB Lightning 版本均 &gt;= v4.0.6，无论一个表是否已经创建 TiFlash 副本，你均可以使用 TiDB Lightning 导入数据至该表。但注意此情况会导致 TiDB Lightning 导入数据耗费的时间延长，具体取决于 TiDB Lightning 部署机器的网卡带宽、TiFlash 节点的 CPU 及磁盘负载、TiFlash 副本数等因素。</p></li><li><p>不推荐同步 1000 张以上的表，这会降低 PD 的调度性能。这个限制将在后续版本去除。</p></li></ul><h3 id="查看表同步进度" style="position:relative"><a href="#%E6%9F%A5%E7%9C%8B%E8%A1%A8%E5%90%8C%E6%AD%A5%E8%BF%9B%E5%BA%A6" aria-label="查看表同步进度 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>查看表同步进度</h3><p>可通过如下 SQL 语句查看特定表（通过 WHERE 语句指定，去掉 WHERE 语句则查看所有表）的 TiFlash 副本的状态：</p><pre><code class="language-sql">SELECT * FROM information_schema.tiflash_replica WHERE TABLE_SCHEMA = '&lt;db_name&gt;' and TABLE_NAME = '&lt;table_name&gt;' </code></pre><p>查询结果中：</p><ul><li>AVAILABLE 字段表示该表的 TiFlash 副本是否可用。1 代表可用，0 代表不可用。副本状态为可用之后就不再改变，如果通过 DDL 命令修改副本数则会重新计算同步进度。</li><li>PROGRESS 字段代表同步进度，在 0.0~1.0 之间，1 代表至少 1 个副本已经完成同步。</li></ul><h3 id="加快-tiflash-副本同步速度" style="position:relative"><a href="#%E5%8A%A0%E5%BF%AB-tiflash-%E5%89%AF%E6%9C%AC%E5%90%8C%E6%AD%A5%E9%80%9F%E5%BA%A6" aria-label="加快 tiflash 副本同步速度 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>加快 TiFlash 副本同步速度</h3><p>新增 TiFlash 副本时，各个 TiKV 实例将进行全表数据扫描，并将扫描得到的数据快照发送给 TiFlash 从而形成副本。默认情况下，为了降低对 TiKV 及 TiFlash 线上业务的影响，TiFlash 新增副本速度较慢、占用资源较少。如果集群中 TiKV 及 TiFlash 的 CPU 和磁盘 IO 资源有富余，你可以按以下步骤操作来提升 TiFlash 副本同步速度：</p><ol><li><p>修改 TiFlash Proxy 及 TiKV 配置，临时调高各个 TiKV 及 TiFlash 实例的数据快照写入速度及快照处理速度。以 TiUP 为例，需要修改的配置项如下：</p><pre><code class="language-yaml">tikv: server.snap-max-write-bytes-per-sec: 300MiB # 默认 100MiB tiflash-learner: raftstore.snap-handle-pool-size: 10 # 默认 2，可以调整为机器总 CPU 数 × 0.6 或更高 raftstore.apply-low-priority-pool-size: 10 # 默认 1，可以调整为机器总 CPU 数 × 0.6 或更高 server.snap-max-write-bytes-per-sec: 300MiB # 默认 100MiB </code></pre><p>上述配置需要重启 TiFlash 及 TiKV 生效。其中 TiKV 配置也可以通过 <a href="/zh/tidb/v5.0/dynamic-config">SQL 语句在线修改</a>，无需重启 TiKV 即可生效：</p><pre><code class="language-sql">SET CONFIG tikv `server.snap-max-write-bytes-per-sec` = '300MiB'; </code></pre><p>由于副本同步速度还受到 PD 副本速度控制，因此以上配置修改完毕后，当前你还无法立即观察到副本同步速度提升。</p></li><li><p>使用 <a href="/zh/tidb/v5.0/pd-control">PD Control</a> 逐步放开新增副本速度限制：</p><p>TiFlash 默认新增副本速度是 30（每分钟大约 30 个 Region 将会新增 TiFlash 副本）。执行以下命令将调整所有 TiFlash 实例的新增副本速度到 60，即原来的 2 倍速度：</p><pre><code class="language-shell">tiup ctl:v&lt;CLUSTER_VERSION&gt; pd -u http://&lt;PD_ADDRESS&gt;:2379 store limit all engine tiflash 60 add-peer </code></pre><blockquote><p>上述命令中，需要将 <code>&lt;CLUSTER_VERSION&gt;</code> 替换为该集群版本，<code>&lt;PD_ADDRESS&gt;:2379</code> 替换为任一 PD 节点的地址。替换后样例为：</p><pre><code class="language-shell">tiup ctl:v6.1.1 pd -u http://192.168.1.4:2379 store limit all engine tiflash 60 add-peer </code></pre></blockquote><p>执行完毕后，几分钟内，你将观察到 TiFlash 节点的 CPU 及磁盘 IO 资源占用显著提升，TiFlash 将更快地创建副本。同时，TiKV 节点的 CPU 及磁盘 IO 资源占用也将有所上升。</p><p>如果此时 TiKV 及 TiFlash 节点的资源仍有富余，且线上业务的延迟没有显著上升，则可以考虑进一步放开调度速度，例如将新增副本的速度增加为原来的 3 倍：</p><pre><code class="language-shell">tiup ctl:v&lt;CLUSTER_VERSION&gt; pd -u http://&lt;PD_ADDRESS&gt;:2379 store limit all engine tiflash 90 add-peer </code></pre></li><li><p>在副本同步完毕后，恢复到默认配置，减少在线业务受到的影响。</p><p>执行以下 PD Control 命令可恢复默认的新增副本速度：</p><pre><code class="language-shell">tiup ctl:v&lt;CLUSTER_VERSION&gt; pd -u http://&lt;PD_ADDRESS&gt;:2379 store limit all engine tiflash 30 add-peer </code></pre><p>在 TiUP 中注释掉新增的配置项，使其恢复到默认值：</p><pre><code class="language-yaml"># tikv: # server.snap-max-write-bytes-per-sec: 300MiB # tiflash-learner: # raftstore.snap-handle-pool-size: 10 # raftstore.apply-low-priority-pool-size: 10 # server.snap-max-write-bytes-per-sec: 300MiB </code></pre></li></ol><h3 id="设置可用区" style="position:relative"><a href="#%E8%AE%BE%E7%BD%AE%E5%8F%AF%E7%94%A8%E5%8C%BA" aria-label="设置可用区 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>设置可用区</h3><p>在配置副本时，如果为了考虑容灾，需要将 TiFlash 的不同数据副本分布到多个数据中心，则可以按如下步骤进行配置：</p><ol><li><p>在集群配置文件中为 TiFlash 节点指定 label. </p><pre><code>tiflash_servers: - host: 172.16.5.81 config: flash.proxy.labels: zone=z1 - host: 172.16.5.82 config: flash.proxy.labels: zone=z1 - host: 172.16.5.85 config: flash.proxy.labels: zone=z2 </code></pre></li><li><p>启动集群后，在创建副本时为副本调度指定 label，语法如下：</p><pre><code class="language-sql">ALTER TABLE table_name SET TIFLASH REPLICA count LOCATION LABELS location_labels </code></pre><p>例如：</p><pre><code class="language-sql">ALTER TABLE t SET TIFLASH REPLICA 2 LOCATION LABELS "zone"; </code></pre></li><li><p>此时 PD 会根据设置的 label 进行调度，将表 <code>t</code> 的两个副本分别调度到两个可用区中。可以通过监控或 pd-ctl 来验证这一点：</p><pre><code class="language-shell">&gt; tiup ctl:&lt;version&gt; pd -u&lt;pd-host&gt;:&lt;pd-port&gt; store "address": "172.16.5.82:23913", "labels": [ { "key": "engine", "value": "tiflash"}, { "key": "zone", "value": "z1" } "region_count": 4, "address": "172.16.5.81:23913", "labels": [ { "key": "engine", "value": "tiflash"}, { "key": "zone", "value": "z1" } "region_count": 5, "address": "172.16.5.85:23913", "labels": [ { "key": "engine", "value": "tiflash"}, { "key": "zone", "value": "z2" } "region_count": 9, </code></pre></li></ol><p>关于使用 label 进行副本调度划分可用区的更多内容，可以参考<a href="/zh/tidb/v5.0/schedule-replicas-by-topology-labels">通过拓扑 label 进行副本调度</a>，<a href="/zh/tidb/v5.0/multi-data-centers-in-one-city-deployment">同城多数据中心部署 TiDB</a> 与<a href="/zh/tidb/v5.0/three-data-centers-in-two-cities-deployment">两地三中心部署</a>。</p><h2 id="使用-tidb-读取-tiflash" style="position:relative"><a href="#%E4%BD%BF%E7%94%A8-tidb-%E8%AF%BB%E5%8F%96-tiflash" aria-label="使用 tidb 读取 tiflash 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>使用 TiDB 读取 TiFlash</h2><p>TiDB 提供三种读取 TiFlash 副本的方式。如果添加了 TiFlash 副本，而没有做任何 engine 的配置，则默认使用 CBO 方式。</p><h3 id="智能选择" style="position:relative"><a href="#%E6%99%BA%E8%83%BD%E9%80%89%E6%8B%A9" aria-label="智能选择 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>智能选择</h3><p>对于创建了 TiFlash 副本的表，TiDB 优化器会自动根据代价估算选择是否使用 TiFlash 副本。具体有没有选择 TiFlash 副本，可以通过 <code>desc</code> 或 <code>explain analyze</code> 语句查看，例如：</p><pre><code class="language-sql">desc select count(*) from test.t; </code></pre><pre><code>+--------------------------+---------+--------------+---------------+--------------------------------+ | id | estRows | task | access object | operator info | +--------------------------+---------+--------------+---------------+--------------------------------+ | StreamAgg_9 | 1.00 | root | | funcs:count(1)-&gt;Column#4 | | └─TableReader_17 | 1.00 | root | | data:TableFullScan_16 | | └─TableFullScan_16 | 1.00 | cop[tiflash] | table:t | keep order:false, stats:pseudo | +--------------------------+---------+--------------+---------------+--------------------------------+ 3 rows in set (0.00 sec) </code></pre><pre><code class="language-sql">explain analyze select count(*) from test.t; </code></pre><pre><code>+--------------------------+---------+---------+--------------+---------------+----------------------------------------------------------------------+--------------------------------+-----------+------+ | id | estRows | actRows | task | access object | execution info | operator info | memory | disk | +--------------------------+---------+---------+--------------+---------------+----------------------------------------------------------------------+--------------------------------+-----------+------+ | StreamAgg_9 | 1.00 | 1 | root | | time:83.8372ms, loops:2 | funcs:count(1)-&gt;Column#4 | 372 Bytes | N/A | | └─TableReader_17 | 1.00 | 1 | root | | time:83.7776ms, loops:2, rpc num: 1, rpc time:83.5701ms, proc keys:0 | data:TableFullScan_16 | 152 Bytes | N/A | | └─TableFullScan_16 | 1.00 | 1 | cop[tiflash] | table:t | time:43ms, loops:1 | keep order:false, stats:pseudo | N/A | N/A | +--------------------------+---------+---------+--------------+---------------+----------------------------------------------------------------------+--------------------------------+-----------+------+ </code></pre><p><code>cop[tiflash]</code> 表示该任务会发送至 TiFlash 进行处理。如果没有选择 TiFlash 副本，可尝试通过 <code>analyze table</code> 语句更新统计信息后，再查看 <code>explain analyze</code> 结果。</p><p>需要注意的是，如果表仅有单个 TiFlash 副本且相关节点无法服务，智能选择模式下的查询会不断重试，需要指定 Engine 或者手工 Hint 来读取 TiKV 副本。</p><h3 id="engine-隔离" style="position:relative"><a href="#engine-%E9%9A%94%E7%A6%BB" aria-label="engine 隔离 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>Engine 隔离</h3><p>Engine 隔离是通过配置变量来指定所有的查询均使用指定 engine 的副本，可选 engine 为 "tikv"、"tidb" 和 "tiflash"（其中 "tidb" 表示 TiDB 内部的内存表区，主要用于存储一些 TiDB 系统表，用户不能主动使用），分别有 2 个配置级别：</p><ol><li><p>TiDB 实例级别，即 INSTANCE 级别。在 TiDB 的配置文件添加如下配置项：</p><pre><code>[isolation-read] engines = ["tikv", "tidb", "tiflash"] </code></pre><p><strong>实例级别的默认配置为 <code>["tikv", "tidb", "tiflash"]</code>。</strong></p></li><li><p>会话级别，即 SESSION 级别。设置语句：</p><pre><code class="language-sql">set @@session.tidb_isolation_read_engines = "逗号分隔的 engine list"; </code></pre><p>或者</p><pre><code class="language-sql">set SESSION tidb_isolation_read_engines = "逗号分隔的 engine list"; </code></pre><p><strong>会话级别的默认配置继承自 TiDB 实例级别的配置。</strong></p></li></ol><p>最终的 engine 配置为会话级别配置，即会话级别配置会覆盖实例级别配置。比如实例级别配置了 "tikv"，而会话级别配置了 "tiflash"，则会读取 TiFlash 副本。当 engine 配置为 "tikv, tiflash"，即可以同时读取 TiKV 和 TiFlash 副本，优化器会自动选择。</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>由于 <a href="/zh/tidb/v5.0/dashboard-intro">TiDB Dashboard</a> 等组件需要读取一些存储于 TiDB 内存表区的系统表，因此建议实例级别 engine 配置中始终加入 "tidb" engine。</p></div></div></div><p>如果查询中的表没有对应 engine 的副本，比如配置了 engine 为 "tiflash" 而该表没有 TiFlash 副本，则查询会报该表不存在该 engine 副本的错。</p><h3 id="手工-hint" style="position:relative"><a href="#%E6%89%8B%E5%B7%A5-hint" aria-label="手工 hint 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>手工 Hint</h3><p>手工 Hint 可以在满足 engine 隔离的前提下，强制 TiDB 对于某张或某几张表使用指定的副本，使用方法为：</p><pre><code class="language-sql">select /*+ read_from_storage(tiflash[table_name]) */ ... from table_name; </code></pre><p>如果在查询语句中对表设置了别名，在 Hint 语句中必须使用别名才能使 Hint 生效。比如：</p><pre><code class="language-sql">select /*+ read_from_storage(tiflash[alias_a,alias_b]) */ ... from table_name_1 as alias_a, table_name_2 as alias_b where alias_a.column_1 = alias_b.column_2; </code></pre><p>其中 <code>tiflash[]</code> 是提示优化器读取 TiFlash 副本，亦可以根据需要使用 <code>tikv[]</code> 来提示优化器读取 TiKV 副本。更多关于该 Hint 语句的语法可以参考 <a href="/zh/tidb/v5.0/optimizer-hints#read_from_storagetiflasht1_name--tl_name--tikvt2_name--tl_name-">READ_FROM_STORAGE</a>。</p><p>如果 Hint 指定的表在指定的引擎上不存在副本，则 Hint 会被忽略，并产生 warning。另外 Hint 必须在满足 engine 隔离的前提下才会生效，如果 Hint 中指定的引擎不在 engine 隔离列表中，Hint 同样会被忽略，并产生 warning。</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>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 <code>Hint</code> 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></div></div></div><h3 id="三种方式之间关系的总结" style="position:relative"><a href="#%E4%B8%89%E7%A7%8D%E6%96%B9%E5%BC%8F%E4%B9%8B%E9%97%B4%E5%85%B3%E7%B3%BB%E7%9A%84%E6%80%BB%E7%BB%93" aria-label="三种方式之间关系的总结 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>三种方式之间关系的总结</h3><p>上述三种读取 TiFlash 副本的方式中，Engine 隔离规定了总的可使用副本 engine 的范围，手工 Hint 可以在该范围内进一步实现语句级别及表级别的细粒度的 engine 指定，最终由 CBO 在指定的 engine 范围内根据代价估算最终选取某个 engine 上的副本。</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>TiDB 4.0.3 版本之前，在非只读 SQL 语句中（比如 <code>INSERT INTO ... SELECT</code>、<code>SELECT ... FOR UPDATE</code>、<code>UPDATE ...</code>、<code>DELETE ...</code>）读取 TiFlash，行为是未定义。TiDB 4.0.3 以及后续的版本，TiDB 内部会对非只读 SQL 语句忽略 TiFlash 副本以保证数据写入、更新、删除的正确性。对应的，如果使用了<a href="#%E6%99%BA%E8%83%BD%E9%80%89%E6%8B%A9">智能选择</a>的方式，TiDB 会自动选择非 TiFlash 副本；如果使用了 <a href="#engine-%E9%9A%94%E7%A6%BB">Engine 隔离</a>的方式指定<strong>仅</strong>读取 TiFlash 副本，则查询会报错；而如果使用了<a href="#%E6%89%8B%E5%B7%A5-hint">手工 Hint</a> 的方式，则 Hint 会被忽略。</p></div></div></div><h2 id="使用-tispark-读取-tiflash" style="position:relative"><a href="#%E4%BD%BF%E7%94%A8-tispark-%E8%AF%BB%E5%8F%96-tiflash" aria-label="使用 tispark 读取 tiflash 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>使用 TiSpark 读取 TiFlash</h2><p>TiSpark 目前提供类似 TiDB 中 engine 隔离的方式读取 TiFlash，方式是通过配置参数 <code>spark.tispark.isolation_read_engines</code>。参数值默认为 <code>tikv,tiflash</code>，表示根据 CBO 自动选择从 TiFlash 或从 TiKV 读取数据。如果将该参数值设置成 <code>tiflash</code>，表示强制从 TiFlash 读取数据。</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>设为 <code>tiflash</code> 时，所有查询的表都会只读取 TiFlash 副本，设为 <code>tikv</code> 则只读取 TiKV 副本。设为 <code>tiflash</code> 时，要求查询所用到的表都必须已创建了 TiFlash 副本，对于未创建 TiFlash 副本的表的查询会报错。</p></div></div></div><p>可以使用以下任意一种方式进行设置：</p><ol><li><p>在 <code>spark-defaults.conf</code> 文件中添加：</p><pre><code>spark.tispark.isolation_read_engines tiflash </code></pre></li><li><p>在启动 Spark shell 或 Thrift server 时，启动命令中添加 <code>--conf spark.tispark.isolation_read_engines=tiflash</code></p></li><li><p>Spark shell 中实时设置：<code>spark.conf.set("spark.tispark.isolation_read_engines", "tiflash")</code></p></li><li><p>Thrift server 通过 beeline 连接后实时设置：<code>set spark.tispark.isolation_read_engines=tiflash</code></p></li></ol><h2 id="tiflash-支持的计算下推" style="position:relative"><a href="#tiflash-%E6%94%AF%E6%8C%81%E7%9A%84%E8%AE%A1%E7%AE%97%E4%B8%8B%E6%8E%A8" aria-label="tiflash 支持的计算下推 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>TiFlash 支持的计算下推</h2><p>TiFlash 支持部分算子的下推，支持的算子如下：</p><ul><li>TableScan：该算子从表中读取数据</li><li>Selection：该算子对数据进行过滤</li><li>HashAgg：该算子基于 <a href="/zh/tidb/v5.0/explain-aggregation#hash-aggregation">Hash Aggregation</a> 算法对数据进行聚合运算</li><li>StreamAgg：该算子基于 <a href="/zh/tidb/v5.0/explain-aggregation#stream-aggregation">Stream Aggregation</a> 算法对数据进行聚合运算。StreamAgg 仅支持不带 <code>GROUP BY</code> 条件的列。</li><li>TopN：该算子对数据求 TopN 运算</li><li>Limit：该算子对数据进行 limit 运算</li><li>Project：该算子对数据进行投影运算</li><li>HashJoin：该算子基于 <a href="/zh/tidb/v5.0/explain-joins#hash-join">Hash Join</a> 算法对数据进行连接运算，但有以下使用条件：<ul><li>只有在 <a href="#%E4%BD%BF%E7%94%A8-mpp-%E6%A8%A1%E5%BC%8F">MPP 模式</a>下才能被下推</li><li>必须带有等值的 join 条件</li><li>不支持下推 <code>Full Outer Join</code></li></ul></li></ul><p>在 TiDB 中，算子之间会呈现树型组织结构。一个算子能下推到 TiFlash 的前提条件，是该算子的所有子算子都能下推到 TiFlash。因为大部分算子都包含有表达式计算，当且仅当一个算子所包含的所有表达式均支持下推到 TiFlash 时，该算子才有可能下推给 TiFlash。目前 TiFlash 支持下推的表达式包括：</p><ul><li>数学函数：<code>+, -, /, *, &gt;=, &lt;=, =, !=, &lt;, &gt;, round(int), round(double), abs, floor(int), ceil(int), ceiling(int)</code></li><li>逻辑函数：<code>and, or, not, case when, if, ifnull, isnull, in</code></li><li>位运算：<code>bitand, bitor, bigneg, bitxor</code></li><li>字符串函数：<code>substr, char_length, replace, concat, concat_ws, left, right</code></li><li>日期函数：<code>date_format, timestampdiff, from_unixtime, unix_timestamp(int), unix_timestamp(decimal), str_to_date(date), str_to_date(datetime), datediff, year, month, day, extract(datetime)</code></li><li>JSON 函数：<code>json_length</code></li><li>转换函数：<code>cast(int as double), cast(int as decimal), cast(int as string), cast(int as time), cast(double as int), cast(double as decimal), cast(double as string), cast(double as time), cast(string as int), cast(string as double), cast(string as decimal), cast(string as time), cast(decimal as int), cast(decimal as string), cast(decimal as time), cast(time as int), cast(time as decimal), cast(time as string)</code></li><li>聚合函数：<code>min, max, sum, count, avg, approx_count_distinct</code></li></ul><p>另外，所有包含 Time/Bit/Set/Enum/Geometry 类型的表达式均不能下推到 TiFlash。</p><p>如查询遇到不支持的下推计算，则需要依赖 TiDB 完成剩余计算，可能会很大程度影响 TiFlash 加速效果。对于暂不支持的算子/表达式，将会在后续版本中陆续提供支持，也可以联系官方沟通。</p><h2 id="使用-mpp-模式" style="position:relative"><a href="#%E4%BD%BF%E7%94%A8-mpp-%E6%A8%A1%E5%BC%8F" aria-label="使用 mpp 模式 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>使用 MPP 模式</h2><p>TiFlash 支持 MPP 模式的查询执行，即在计算中引入跨节点的数据交换（data shuffle 过程）。MPP 模式默认开启，如需关闭可将全局/会话变量 <a href="/zh/tidb/v5.0/system-variables#tidb_allow_mpp-从-v50-版本开始引入"><code>tidb_allow_mpp</code></a> 的值设为 <code>0</code> 或 <code>OFF</code>：</p><pre><code class="language-shell">set @@session.tidb_allow_mpp=0 </code></pre><p>MPP 模式目前支持的物理算法有：Broadcast Hash Join、Shuffled Hash Join 和 Shuffled Hash Aggregation。算法的选择由优化器自动判断。通过 <code>EXPLAIN</code> 语句可以查看具体的查询执行计划。如果 <code>EXPLAIN</code> 语句的结果中出现 ExchangeSender 和 ExchangeReceiver 算子，表明 MPP 已生效。</p><p>以 TPC-H 测试集中的表结构为例：</p><pre><code class="language-sql">mysql&gt; explain select count(*) from customer c join nation n on c.c_nationkey=n.n_nationkey; +------------------------------------------+------------+-------------------+---------------+----------------------------------------------------------------------------+ | id | estRows | task | access object | operator info | +------------------------------------------+------------+-------------------+---------------+----------------------------------------------------------------------------+ | HashAgg_23 | 1.00 | root | | funcs:count(Column#16)-&gt;Column#15 | | └─TableReader_25 | 1.00 | root | | data:ExchangeSender_24 | | └─ExchangeSender_24 | 1.00 | batchCop[tiflash] | | ExchangeType: PassThrough | | └─HashAgg_12 | 1.00 | batchCop[tiflash] | | funcs:count(1)-&gt;Column#16 | | └─HashJoin_17 | 3000000.00 | batchCop[tiflash] | | inner join, equal:[eq(tpch.nation.n_nationkey, tpch.customer.c_nationkey)] | | ├─ExchangeReceiver_21(Build) | 25.00 | batchCop[tiflash] | | | | │ └─ExchangeSender_20 | 25.00 | batchCop[tiflash] | | ExchangeType: Broadcast | | │ └─TableFullScan_18 | 25.00 | batchCop[tiflash] | table:n | keep order:false | | └─TableFullScan_22(Probe) | 3000000.00 | batchCop[tiflash] | table:c | keep order:false | +------------------------------------------+------------+-------------------+---------------+----------------------------------------------------------------------------+ 9 rows in set (0.00 sec) </code></pre><p>在执行计划中，出现了 <code>ExchangeReceiver</code> 和 <code>ExchangeSender</code> 算子。该执行计划表示 <code>nation</code> 表读取完毕后，经过 <code>ExchangeSender</code> 算子广播到各个节点中，与 <code>customer</code> 表先后进行 <code>HashJoin</code> 和 <code>HashAgg</code> 操作，再将结果返回至 TiDB 中。</p><p>TiFlash 提供了两个全局/会话变量决定是否选择 Broadcast Hash Join，分别为：</p><ul><li><a href="/zh/tidb/v5.0/system-variables#tidb_broadcast_join_threshold_count-从-v50-版本开始引入"><code>tidb_broadcast_join_threshold_size</code></a>，单位为 bytes。如果表大小（字节数）小于该值，则选择 Broadcast Hash Join 算法。否则选择 Shuffled Hash Join 算法。</li><li><a href="/zh/tidb/v5.0/system-variables#tidb_broadcast_join_threshold_count-从-v50-版本开始引入"><code>tidb_broadcast_join_threshold_count</code></a>，单位为行数。如果 join 的对象为子查询，优化器无法估计子查询结果集大小，在这种情况下通过结果集行数判断。如果子查询的行数估计值小于该变量，则选择 Broadcast Hash Join 算法。否则选择 Shuffled Hash Join 算法。</li></ul><h2 id="注意事项" style="position:relative"><a href="#%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9" aria-label="注意事项 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>注意事项</h2><p>TiFlash 目前尚不支持的一些功能，与原生 TiDB 可能存在不兼容的问题，具体如下：</p><ul><li><p>TiFlash 计算层：</p><ul><li><p>不支持检查溢出的数值。例如将两个 <code>BIGINT</code> 类型的最大值相加 <code>9223372036854775807 + 9223372036854775807</code>，该计算在 TiDB 中预期的行为是返回错误 <code>ERROR 1690 (22003): BIGINT value is out of range</code>，但如果该计算在 TiFlash 中进行，则会得到溢出的结果 <code>-2</code> 且无报错。</p></li><li><p>不支持窗口函数。</p></li><li><p>不支持从 TiKV 读取数据。</p></li><li><p>目前 TiFlash 中的 <code>sum</code> 函数不支持传入字符串类型的参数，但 TiDB 在编译时无法检测出这种情况。所以当执行类似于 <code>select sum(string_col) from t</code> 的语句时，TiFlash 会报错 <code>[FLASH:Coprocessor:Unimplemented] CastStringAsReal is not supported.</code>。要避免这类报错，需要手动把 SQL 改写成 <code>select sum(cast(string_col as double)) from t</code>。</p></li><li><p>TiFlash 目前的 Decimal 除法计算和 TiDB 存在不兼容的情况。例如在进行 Decimal 相除的时候，TiFlash 会始终按照编译时推断出来的类型进行计算，而 TiDB 则在计算过程中采用精度高于编译时推断出来的类型。这导致在一些带有 Decimal 除法的 SQL 语句在 TiDB + TiKV 上的执行结果会和 TiDB + TiFlash 上的执行结果不一样，示例如下：</p><pre><code class="language-sql">mysql&gt; create table t (a decimal(3,0), b decimal(10, 0)); Query OK, 0 rows affected (0.07 sec) mysql&gt; insert into t values (43, 1044774912); Query OK, 1 row affected (0.03 sec) mysql&gt; alter table t set tiflash replica 1; Query OK, 0 rows affected (0.07 sec) mysql&gt; set session tidb_isolation_read_engines='tikv'; Query OK, 0 rows affected (0.00 sec) mysql&gt; select a/b, a/b + 0.0000000000001 from t where a/b; +--------+-----------------------+ | a/b | a/b + 0.0000000000001 | +--------+-----------------------+ | 0.0000 | 0.0000000410001 | +--------+-----------------------+ 1 row in set (0.00 sec)