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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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-indexes"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">索引查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-4" 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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-joins"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">Join 查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-5" 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-5fc025"><a aria-current="page" style="text-decoration:none;width:100%;color:inherit" class="" href="/zh/tidb/stable/explain-subqueries"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">子查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-6" 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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-aggregation"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">聚合查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-7" 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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-views"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">视图查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-8" 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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-partitions"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">分区查询的执行计划</div></div></a></div></div></div></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" id="left-nav-treeview-0-10-2-1-9" 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-5fc025"><a style="text-decoration:none;width:100%;color:inherit" href="/zh/tidb/stable/explain-index-merge"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">开启 IndexMerge 查询的执行计划</div></div></a></div></div></div></li></div></div></ul></li><li 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class="MuiTreeItem-iconContainer"/><div class="MuiTreeItem-label"><div class="css-snygwd"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">控制执行计划</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></div></div></ul></li></div></div></ul></li><li class="MuiTreeItem-root css-1oy5rnm" role="treeitem" aria-expanded="false" id="left-nav-treeview-0-11" 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-znl8a3"><div class="MuiBox-root css-8atqhb"><div class="MuiTypography-root MuiTypography-body1 css-1hwyipb">教程</div></div><svg class="MuiSvgIcon-root 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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-0-16" 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/stable/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 css-1rnnmjb"><style data-emotion="css 90fn5w">.css-90fn5w{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;}@media (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="用-explain-查看子查询的执行计划" style="position:relative"><a href="#%E7%94%A8-explain-%E6%9F%A5%E7%9C%8B%E5%AD%90%E6%9F%A5%E8%AF%A2%E7%9A%84%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92" aria-label="用 explain 查看子查询的执行计划 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>用 EXPLAIN 查看子查询的执行计划</h1><p>TiDB 会执行多种<a href="/zh/tidb/stable/subquery-optimization">子查询相关的优化</a>，以提升子查询的执行性能。本文档介绍一些常见子查询的优化方式，以及如何解读 <code>EXPLAIN</code> 语句返回的执行计划信息。</p><p>本文档所使用的示例表数据如下：</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"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t1 (id <span class="hljs-type">BIGINT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span> <span class="hljs-keyword">PRIMARY</span> KEY auto_increment, pad1 <span class="hljs-type">BLOB</span>, pad2 <span class="hljs-type">BLOB</span>, pad3 <span class="hljs-type">BLOB</span>, int_col <span class="hljs-type">INT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span> <span class="hljs-keyword">DEFAULT</span> <span class="hljs-number">0</span>); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t2 (id <span class="hljs-type">BIGINT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span> <span class="hljs-keyword">PRIMARY</span> KEY auto_increment, t1_id <span class="hljs-type">BIGINT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span>, pad1 <span class="hljs-type">BLOB</span>, pad2 <span class="hljs-type">BLOB</span>, pad3 <span class="hljs-type">BLOB</span>, INDEX(t1_id)); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t3 ( id <span class="hljs-type">INT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span> <span class="hljs-keyword">PRIMARY</span> KEY auto_increment, t1_id <span class="hljs-type">INT</span> <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">NULL</span>, <span class="hljs-keyword">UNIQUE</span> (t1_id) <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> dual; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t1 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), <span class="hljs-number">0</span> <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t2 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, a.id, RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>), RANDOM_BYTES(<span class="hljs-number">1024</span>) <span class="hljs-keyword">FROM</span> t1 a <span class="hljs-keyword">JOIN</span> t1 b <span class="hljs-keyword">JOIN</span> t1 c LIMIT <span class="hljs-number">10000</span>; <span class="hljs-keyword">UPDATE</span> t1 <span class="hljs-keyword">SET</span> int_col <span class="hljs-operator">=</span> <span class="hljs-number">1</span> <span class="hljs-keyword">WHERE</span> pad1 <span class="hljs-operator">=</span> (<span class="hljs-keyword">SELECT</span> pad1 <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">ORDER</span> <span class="hljs-keyword">BY</span> RAND() LIMIT <span class="hljs-number">1</span>); <span class="hljs-keyword">INSERT</span> <span class="hljs-keyword">INTO</span> t3 <span class="hljs-keyword">SELECT</span> <span class="hljs-keyword">NULL</span>, id <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> id <span class="hljs-operator">&lt;</span> <span class="hljs-number">1000</span>; <span class="hljs-keyword">SELECT</span> SLEEP(<span class="hljs-number">1</span>); ANALYZE <span class="hljs-keyword">TABLE</span> t1, t2, t3; </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"><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="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><h2 id="inner-join无-unique-约束的子查询" style="position:relative"><a href="#inner-join%E6%97%A0-unique-%E7%BA%A6%E6%9D%9F%E7%9A%84%E5%AD%90%E6%9F%A5%E8%AF%A2" aria-label="inner join无 unique 约束的子查询 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>Inner join（无 <code>UNIQUE</code> 约束的子查询）</h2><p>以下示例中，<code>IN</code> 子查询会从表 <code>t2</code> 中搜索一列 ID。为保证语义正确性，TiDB 需要保证 <code>t1_id</code> 列的值具有唯一性。使用 <code>EXPLAIN</code> 可查看到该查询的执行计划去掉重复项并执行 <code>Inner Join</code> 内连接操作：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> id <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> t1_id <span class="hljs-keyword">FROM</span> t2); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_15 <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_12, <span class="hljs-keyword">outer</span> key:test.t2.t1_id, <span class="hljs-keyword">inner</span> key:test.t1.id, equal cond:eq(test.t2.t1_id, test.t1.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─StreamAgg_44(Build) <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.t2.t1_id, funcs:firstrow(test.t2.t1_id)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span>test.t2.t1_id <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexReader_45 <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:StreamAgg_34 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─StreamAgg_34 <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.t2.t1_id, <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_26 <span class="hljs-operator">|</span> <span class="hljs-number">90000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2, index:t1_id(t1_id) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_12(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_11 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_11 <span class="hljs-operator">|</span> <span class="hljs-number">21.11</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t2.t1_id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>由上述查询结果可知，TiDB 通过索引连接操作 <code>| IndexJoin_14</code> 将子查询做了连接转化。该执行计划首先在 TiKV 侧通过索引扫描算子 <code>└─IndexFullScan_31</code> 读取 <code>t2.t1_id</code> 列的值，然后由 <code>└─StreamAgg_39</code> 算子的部分任务在 TiKV 中对 <code>t1_id</code> 值进行去重，然后采用 <code>├─StreamAgg_49(Build)</code> 算子的部分任务在 TiDB 中对 <code>t1_id</code> 值再次进行去重，去重操作由聚合函数 <code>firstrow(test.t2.t1_id)</code> 执行；之后将操作结果与 <code>t1</code> 表的主键相连接，连接条件是 <code>eq(test.t1.id, test.t2.t1_id)</code>。</p><h2 id="inner-join有-unique-约束的子查询" style="position:relative"><a href="#inner-join%E6%9C%89-unique-%E7%BA%A6%E6%9D%9F%E7%9A%84%E5%AD%90%E6%9F%A5%E8%AF%A2" aria-label="inner join有 unique 约束的子查询 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>Inner join（有 <code>UNIQUE</code> 约束的子查询）</h2><p>在上述示例中，为了确保 <code>t1_id</code> 值在与表 <code>t1</code> 连接前具有唯一性，需要执行聚合运算。在以下示例中，由于 <code>UNIQUE</code> 约束已能确保 <code>t3.t1_id</code> 列值的唯一：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> id <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> t1_id <span class="hljs-keyword">FROM</span> t3); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_18 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_15, <span class="hljs-keyword">outer</span> key:test.t3.t1_id, <span class="hljs-keyword">inner</span> key:test.t1.id, equal cond:eq(test.t3.t1_id, test.t1.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexReader_41(Build) <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_40 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_40 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t3, index:t1_id(t1_id) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_15(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableRangeScan_14 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_14 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t3.t1_id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>从语义上看，因为约束保证了 <code>t3.t1_id</code> 列值的唯一性，TiDB 可以直接执行 <code>INNER JOIN</code> 查询。</p><h2 id="semi-join关联查询" style="position:relative"><a href="#semi-join%E5%85%B3%E8%81%94%E6%9F%A5%E8%AF%A2" aria-label="semi join关联查询 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>Semi Join（关联查询）</h2><p>在前两个示例中，通过 <code>StreamAgg</code> 聚合操作或通过 <code>UNIQUE</code> 约束保证子查询数据的唯一性之后，TiDB 才能够执行 <code>Inner Join</code> 操作。这两种连接均使用了 <code>Index Join</code>。</p><p>下面的例子中，TiDB 优化器则选择了一种不同的执行计划：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> id <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> t1_id <span class="hljs-keyword">FROM</span> t2 <span class="hljs-keyword">WHERE</span> t1_id <span class="hljs-operator">!=</span> t1.int_col); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> MergeJoin_9 <span class="hljs-operator">|</span> <span class="hljs-number">45446.40</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> semi <span class="hljs-keyword">join</span>, <span class="hljs-keyword">left</span> key:test.t1.id, <span class="hljs-keyword">right</span> key:test.t2.t1_id, other cond:ne(test.t2.t1_id, test.t1.int_col) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─IndexReader_24(Build) <span class="hljs-operator">|</span> <span class="hljs-number">90000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> index:IndexFullScan_23 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─IndexFullScan_23 <span class="hljs-operator">|</span> <span class="hljs-number">90000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t2, index:t1_id(t1_id) <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_22(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">56808.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:Selection_21 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Selection_21 <span class="hljs-operator">|</span> <span class="hljs-number">56808.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ne(test.t1.id, test.t1.int_col) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_20 <span class="hljs-operator">|</span> <span class="hljs-number">71010.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">true</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>由上述查询结果可知，TiDB 执行了 <code>Semi Join</code>。不同于 <code>Inner Join</code>，<code>Semi Join</code> 仅允许右键 (<code>t2.t1_id</code>) 上的第一个值，也就是该操作将去除 <code>Join</code> 算子任务中的重复数据。<code>Join</code> 算法也包含 <code>Merge Join</code>，会按照排序顺序同时从左侧和右侧读取数据，这是一种高效的 <code>Zipper Merge</code>。</p><p>可以将原语句视为<em>关联子查询</em>，因为它引入了子查询外的 <code>t1.int_col</code> 列。然而，<code>EXPLAIN</code> 语句的返回结果显示的是<a href="/zh/tidb/stable/correlated-subquery-optimization">关联子查询去关联</a>后的执行计划。条件 <code>t1_id != t1.int_col</code> 会被重写为 <code>t1.id != t1.int_col</code>。TiDB 可以从表 <code>t1</code> 中读取数据并且在 <code>└─Selection_21</code> 中执行此操作，因此这种去关联和重写操作会极大提高执行效率。</p><h2 id="anti-semi-joinnot-in-子查询" style="position:relative"><a href="#anti-semi-joinnot-in-%E5%AD%90%E6%9F%A5%E8%AF%A2" aria-label="anti semi joinnot in 子查询 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>Anti Semi Join（<code>NOT IN</code> 子查询）</h2><p>在以下示例中，<em>除非</em>子查询中存在 <code>t3.t1_id</code>，否则该查询将（从语义上）返回表 <code>t3</code> 中的所有行：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t3 <span class="hljs-keyword">WHERE</span> t1_id <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> id <span class="hljs-keyword">FROM</span> t1 <span class="hljs-keyword">WHERE</span> int_col <span class="hljs-operator">&lt;</span> <span class="hljs-number">100</span>); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> IndexJoin_16 <span class="hljs-operator">|</span> <span class="hljs-number">799.20</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> anti semi <span class="hljs-keyword">join</span>, <span class="hljs-keyword">inner</span>:TableReader_12, <span class="hljs-keyword">outer</span> key:test.t3.t1_id, <span class="hljs-keyword">inner</span> key:test.t1.id, equal cond:eq(test.t3.t1_id, test.t1.id) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_28(Build) <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_27 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_27 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t3 <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span> <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_12(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:Selection_11 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Selection_11 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> lt(test.t1.int_col, <span class="hljs-number">100</span>) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableRangeScan_10 <span class="hljs-operator">|</span> <span class="hljs-number">999.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t1 <span class="hljs-operator">|</span> <span class="hljs-keyword">range</span>: decided <span class="hljs-keyword">by</span> [test.t3.t1_id], keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span> <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------------------------------------------+</span> </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>上述查询首先读取了表 <code>t3</code>，然后根据主键开始探测 (probe) 表 <code>t1</code>。连接类型是 <em>anti semi join</em>，即反半连接：之所以使用 <em>anti</em>，是因为上述示例有不存在匹配值（即 <code>NOT IN</code>）的情况；使用 <code>Semi Join</code> 则是因为仅需要匹配第一行后就可以停止查询。</p><h2 id="null-aware-semi-joinin-和--any-子查询" style="position:relative"><a href="#null-aware-semi-joinin-%E5%92%8C--any-%E5%AD%90%E6%9F%A5%E8%AF%A2" aria-label="null aware semi joinin 和 any 子查询 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>Null-Aware Semi Join（<code>IN</code> 和 <code>= ANY</code> 子查询）</h2><p><code>IN</code> 和 <code>= ANY</code> 的集合运算符号具有特殊的三值属性（<code>true</code>、<code>false</code> 和 <code>NULL</code>）。这意味着在该运算符所转化得到的 Join 类型中需要对 Join key 两侧的 <code>NULL</code> 进行特殊的感知和处理。</p><p><code>IN</code> 和 <code>= ANY</code> 算子引导的子查询会分别转为 Semi Join 和 Left Outer Semi Join。在上述 <a href="#semi-join%E5%85%B3%E8%81%94%E6%9F%A5%E8%AF%A2">Semi Join</a> 小节中，示例中 Join key 两侧的列 <code>test.t1.id</code> 和 <code>test.t2.t1_id</code> 都为 <code>not NULL</code> 属性，所以 Semi Join 本身不需要 Null-Aware 的性质来辅助运算，即不需要特殊处理 <code>NULL</code>。当前 TiDB 对于 Null-Aware Semi Join 没有特定的优化，其实现本质都是基于笛卡尔积加过滤 (filter) 的模式。以下为 Null-Aware Semi Join 的例子：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> s(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>); EXPLAIN <span class="hljs-keyword">SELECT</span> (a,b) <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s) <span class="hljs-keyword">FROM</span> t; EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> (a,b) <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">tidb<span class="hljs-operator">&gt;</span> EXPLAIN <span class="hljs-keyword">SELECT</span> (a,b) <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s) <span class="hljs-keyword">FROM</span> t; <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_8 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> CARTESIAN <span class="hljs-keyword">left</span> <span class="hljs-keyword">outer</span> semi <span class="hljs-keyword">join</span>, other cond:eq(test.t.a, test.s.a), eq(test.t.b, test.s.b) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_12(Build) <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_11 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_11 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:s <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_10(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_9 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_9 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------------+</span> <span class="hljs-number">5</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) tidb<span class="hljs-operator">&gt;</span> EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> (a,b) <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s); <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+---------------+-----------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+---------------+-----------------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_11 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">inner</span> <span class="hljs-keyword">join</span>, equal:[eq(test.t.a, test.s.a) eq(test.t.b, test.s.b)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_14(Build) <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:Selection_13 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─Selection_13 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">not</span>(isnull(test.t.a)), <span class="hljs-keyword">not</span>(isnull(test.t.b)) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_12 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─HashAgg_17(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span>:test.s.a, test.s.b, funcs:firstrow(test.s.a)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span>test.s.a, funcs:firstrow(test.s.b)<span class="hljs-operator">-</span><span class="hljs-operator">&gt;</span>test.s.b <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_24 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:Selection_23 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─Selection_23 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">not</span>(isnull(test.s.a)), <span class="hljs-keyword">not</span>(isnull(test.s.b)) <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_22 <span class="hljs-operator">|</span> <span class="hljs-number">1.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:s <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">------------------------------+---------+-----------+---------------+-----------------------------------------------------------------------------------------------------+</span> <span class="hljs-number">8</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.01</span> sec) </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><p>第一个查询 <code>EXPLAIN SELECT (a,b) IN (SELECT * FROM s) FROM t;</code> 中，由于 <code>t</code> 表和 <code>s</code> 表的 <code>a</code>、<code>b</code> 列都是 NULLABLE 的，所以 <code>IN</code> 子查询所转化的 Left Outer Semi Join 是具有 Null-Aware 性质的。具体实现是先进行笛卡尔积，然后将 <code>IN</code> 或 <code>= ANY</code> 所连接的列作为普通等值条件放到 other condition 进行过滤（filter）。</p><p>第二个查询 <code>EXPLAIN SELECT * FROM t WHERE (a,b) IN (SELECT * FROM s);</code> 中，由于 <code>t</code> 表和 <code>s</code> 表的 <code>a</code>、<code>b</code> 列都是 NULLABLE 的，<code>IN</code> 子查询本应该转为具有 Null-Aware 性质的 Semi Join，但当前 TiDB 进行了优化，直接将 Semi Join 转为了 Inner Join + Aggregate 的方式来实现。这是因为在非 scalar 输出的 <code>IN</code> 子查询中，<code>NULL</code> 和 <code>false</code> 是等效的。下推过滤的 <code>NULL</code> 行导致了 <code>WHERE</code> 子句的否定语义，因此可以事先忽略这些行。</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"><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><code>Exists</code> 操作符也会被转成 Semi Join，但是 <code>Exists</code> 操作符号本身不具有集合运算 Null-Aware 的性质。</p></div></div></div><h2 id="null-aware-anti-semi-joinnot-in-和--all-子查询" style="position:relative"><a href="#null-aware-anti-semi-joinnot-in-%E5%92%8C--all-%E5%AD%90%E6%9F%A5%E8%AF%A2" aria-label="null aware anti semi joinnot in 和 all 子查询 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>Null-Aware Anti Semi Join（<code>NOT IN</code> 和 <code>!= ALL</code> 子查询）</h2><p><code>NOT IN</code> 和 <code>!= ALL</code> 的集合运算运算具有特殊的三值属性（<code>true</code>、<code>false</code> 和 <code>NULL</code>）。这意味着在其所转化得到的 Join 类型中需要对 Join key 两侧的 <code>NULL</code> 进行特殊的感知和处理。</p><p><code>NOT IN</code> 和 <code>!= ALL</code> 算子引导的子查询会对应地转为 Anti Semi Join 和 Anti Left Outer Semi Join。在上述的 <a href="#anti-semi-joinnot-in-%E5%AD%90%E6%9F%A5%E8%AF%A2">Anti Semi Join</a> 小节中，由于示例中 Join key 两侧的列 <code>test.t3.t1_id</code> 和 <code>test.t1.id</code> 都是 <code>not NULL</code> 属性的，所以 Anti Semi Join 本身不需要 Null-Aware 的性质来辅助计算，即不需要特殊处理 <code>NULL</code>。</p><p>在 TiDB v6.3.0 版本，TiDB 引入了针对 Null-Aware Anti Join (NAAJ) 的如下特殊优化：</p><ul><li><p>利用 Null-Aware 的等值条件 (NA-EQ) 构建哈希连接</p><p> 由于集合操作符引入的等值需要对等值两侧操作符数的 <code>NULL</code> 值做特殊处理，这里称需要 Null-Aware 的等值条件为 NA-EQ 条件。与 v6.3.0 之前版本不同的是，TiDB 不会再将 NA-EQ 条件处理成普通 EQ 条件，而是专门放置于 Join 后置的 other condition 中，匹配笛卡尔积后再判断结果集的合法性。</p><p> 在 TiDB v6.3.0 版本中，NA-EQ 这种弱化的等值条件依然会被用来构建哈希值 (Hash Join)，大大减少了匹配时所需遍历的数据量，加速匹配过程。在 build 表 <code>DISTINCT</code> 值比例趋近 1 的时候，加速效果更为显著。</p></li><li><p>利用两侧数据源 <code>NULL</code> 值的特殊性质加速匹配过程的返回</p><p> 由于 Anti Semi Join 自身具有 CNF (Conjunctive normal form) 表达式的属性，其任何一侧出现的 <code>NULL</code> 值都会导致确定的结果。利用这个性质可以来加速整个匹配过程。</p></li></ul><p>以下为 Null-Aware Anti Semi Join 的例子：</p><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia"><span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> t(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>); <span class="hljs-keyword">CREATE</span> <span class="hljs-keyword">TABLE</span> s(a <span class="hljs-type">INT</span>, b <span class="hljs-type">INT</span>); EXPLAIN <span class="hljs-keyword">SELECT</span> (a, b) <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s) <span class="hljs-keyword">FROM</span> t; EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> (a, b) <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s); </div><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-1rmx1rm" tabindex="0" type="button" aria-label="Toggle code wrap"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="WrapTextIcon"><path d="M4 19h6v-2H4v2zM20 5H4v2h16V5zm-3 6H4v2h13.25c1.1 0 2 .9 2 2s-.9 2-2 2H15v-2l-3 3 3 3v-2h2c2.21 0 4-1.79 4-4s-1.79-4-4-4z"/></svg></button><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeSmall css-fxo539" tabindex="0" type="button" aria-label="copy"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></code></pre><pre><code class="hljs language-sql"><div class="MuiBox-root css-1qhimia">tidb<span class="hljs-operator">&gt;</span> EXPLAIN <span class="hljs-keyword">SELECT</span> (a, b) <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s) <span class="hljs-keyword">FROM</span> t; <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_8 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">Null</span><span class="hljs-operator">-</span>aware anti <span class="hljs-keyword">left</span> <span class="hljs-keyword">outer</span> semi <span class="hljs-keyword">join</span>, equal:[eq(test.t.b, test.s.b) eq(test.t.a, test.s.a)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_12(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_11 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> │ └─TableFullScan_11 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:s <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableReader_10(Probe) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_9 <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> └─TableFullScan_9 <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> cop[tikv] <span class="hljs-operator">|</span> <span class="hljs-keyword">table</span>:t <span class="hljs-operator">|</span> keep <span class="hljs-keyword">order</span>:<span class="hljs-literal">false</span>, stats:pseudo <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+</span> <span class="hljs-number">5</span> <span class="hljs-keyword">rows</span> <span class="hljs-keyword">in</span> <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) tidb<span class="hljs-operator">&gt;</span> EXPLAIN <span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> t <span class="hljs-keyword">WHERE</span> (a, b) <span class="hljs-keyword">NOT</span> <span class="hljs-keyword">IN</span> (<span class="hljs-keyword">SELECT</span> <span class="hljs-operator">*</span> <span class="hljs-keyword">FROM</span> s); <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+---------------+----------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> id <span class="hljs-operator">|</span> estRows <span class="hljs-operator">|</span> task <span class="hljs-operator">|</span> access object <span class="hljs-operator">|</span> operator info <span class="hljs-operator">|</span> <span class="hljs-operator">+</span><span class="hljs-comment">-----------------------------+----------+-----------+---------------+----------------------------------------------------------------------------------+</span> <span class="hljs-operator">|</span> HashJoin_8 <span class="hljs-operator">|</span> <span class="hljs-number">8000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> <span class="hljs-keyword">Null</span><span class="hljs-operator">-</span>aware anti semi <span class="hljs-keyword">join</span>, equal:[eq(test.t.b, test.s.b) eq(test.t.a, test.s.a)] <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> ├─TableReader_12(Build) <span class="hljs-operator">|</span> <span class="hljs-number">10000.00</span> <span class="hljs-operator">|</span> root <span class="hljs-operator">|</span> <span class="hljs-operator">|</span> data:TableFullScan_11 <span class="hljs-operator">|</span>