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(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="sql-操作常见问题" style="position:relative"><a href="#sql-%E6%93%8D%E4%BD%9C%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98" aria-label="sql 操作常见问题 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>SQL 操作常见问题</h1><p>本文档介绍 TiDB 中常见的 SQL 操作问题。</p><h2 id="tidb-是否支持二级键" style="position:relative"><a href="#tidb-%E6%98%AF%E5%90%A6%E6%94%AF%E6%8C%81%E4%BA%8C%E7%BA%A7%E9%94%AE" aria-label="tidb 是否支持二级键 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>TiDB 是否支持二级键？</h2><p>支持。你可以在具有唯一<a href="/zh/tidb/stable/dev-guide-create-secondary-indexes">二级索引</a>的非主键列上设置 <a href="/zh/tidb/stable/constraints#非空约束"><code>NOT NULL</code> 约束</a>。在这种情况下，该列用作二级键。</p><h2 id="tidb-在对大表执行-ddl-操作时性能表现如何" style="position:relative"><a href="#tidb-%E5%9C%A8%E5%AF%B9%E5%A4%A7%E8%A1%A8%E6%89%A7%E8%A1%8C-ddl-%E6%93%8D%E4%BD%9C%E6%97%B6%E6%80%A7%E8%83%BD%E8%A1%A8%E7%8E%B0%E5%A6%82%E4%BD%95" aria-label="tidb 在对大表执行 ddl 操作时性能表现如何 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>TiDB 在对大表执行 DDL 操作时，性能表现如何？</h2><p>TiDB 在对大表执行 DDL 操作时，一般不会有什么问题。TiDB 支持在线 DDL 操作，且这些 DDL 操作不会阻塞 DML 操作。</p><p>对于添加列、删除列或删除索引等 DDL 操作，TiDB 可以快速完成这些操作。</p><p>对于添加索引等 DDL 操作，TiDB 需要进行回填 (backfill) 操作，这个过程需要较长的时间（取决于表的大小）和额外的资源消耗。对在线业务的影响可调节。TiDB 可以通过多线程进行 backfill，资源消耗可通过以下系统变量进行设置：</p><ul><li><a href="/zh/tidb/stable/system-variables#tidb_ddl_reorg_worker_cnt"><code>tidb_ddl_reorg_worker_cnt</code></a></li><li><a href="/zh/tidb/stable/system-variables#tidb_ddl_reorg_priority"><code>tidb_ddl_reorg_priority</code></a></li><li><a href="/zh/tidb/stable/system-variables#tidb_ddl_error_count_limit"><code>tidb_ddl_error_count_limit</code></a></li><li><a href="/zh/tidb/stable/system-variables#tidb_ddl_reorg_batch_size"><code>tidb_ddl_reorg_batch_size</code></a></li></ul><h2 id="如何选择正确的查询计划是否需要使用优化器提示还是可以使用提示" style="position:relative"><a href="#%E5%A6%82%E4%BD%95%E9%80%89%E6%8B%A9%E6%AD%A3%E7%A1%AE%E7%9A%84%E6%9F%A5%E8%AF%A2%E8%AE%A1%E5%88%92%E6%98%AF%E5%90%A6%E9%9C%80%E8%A6%81%E4%BD%BF%E7%94%A8%E4%BC%98%E5%8C%96%E5%99%A8%E6%8F%90%E7%A4%BA%E8%BF%98%E6%98%AF%E5%8F%AF%E4%BB%A5%E4%BD%BF%E7%94%A8%E6%8F%90%E7%A4%BA" aria-label="如何选择正确的查询计划是否需要使用优化器提示还是可以使用提示 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>如何选择正确的查询计划？是否需要使用优化器提示？还是可以使用提示？</h2><p>TiDB 包含一个基于成本的优化器。在大多数情况下，优化器会为你选择最优的查询计划。如果优化器工作欠佳，你可以使用<a href="/zh/tidb/stable/optimizer-hints">优化器提示</a>来干预优化器。</p><p>另外，你还可以使用<a href="/zh/tidb/stable/sql-plan-management#执行计划绑定-sql-binding">执行计划绑定</a>来为特定的 SQL 语句固定查询计划。</p><h2 id="如何阻止特定的-sql-语句执行或者将某个-sql-语句加入黑名单" style="position:relative"><a href="#%E5%A6%82%E4%BD%95%E9%98%BB%E6%AD%A2%E7%89%B9%E5%AE%9A%E7%9A%84-sql-%E8%AF%AD%E5%8F%A5%E6%89%A7%E8%A1%8C%E6%88%96%E8%80%85%E5%B0%86%E6%9F%90%E4%B8%AA-sql-%E8%AF%AD%E5%8F%A5%E5%8A%A0%E5%85%A5%E9%BB%91%E5%90%8D%E5%8D%95" aria-label="如何阻止特定的 sql 语句执行或者将某个 sql 语句加入黑名单 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>如何阻止特定的 SQL 语句执行（或者将某个 SQL 语句加入黑名单）？</h2><p>你可以使用 <a href="/zh/tidb/stable/optimizer-hints#max_execution_timen"><code>MAX_EXECUTION_TIME</code></a> Hint 来创建 <a href="/zh/tidb/stable/sql-plan-management#执行计划绑定-sql-binding">SQL 绑定</a>，将特定语句的执行时间限制为一个较小的值（例如 1ms）。这样，语句就会在超过限制时自动终止。</p><p>例如，要阻止执行 <code>SELECT * FROM t1, t2 WHERE t1.id = t2.id</code>，可以使用以下 SQL 绑定将语句的执行时间限制为 1ms：</p><pre><code class="language-sql">CREATE GLOBAL BINDING for SELECT * FROM t1, t2 WHERE t1.id = t2.id USING SELECT /*+ MAX_EXECUTION_TIME(1) */ * FROM t1, t2 WHERE t1.id = t2.id; </code></pre><style data-emotion="css 1weqspv">.css-1weqspv{padding-top:0.5rem;padding-bottom:0.5rem;}.css-1weqspv .MuiAlert-message p{margin-bottom:0px;margin-top:0px;}</style><div class="MuiBox-root css-1weqspv"><style data-emotion="css 17zlexa">.css-17zlexa{font-size:0.875rem;line-height:1.25rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:rgb(1, 67, 97);background-color:rgb(229, 246, 253);display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;padding:6px 16px;}.css-17zlexa .MuiAlert-icon{color:#03a9f4;}</style><style data-emotion="css 152gid5">.css-152gid5{background-color:#fff;color:rgba(0, 0, 0, 0.87);-webkit-transition:box-shadow 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:box-shadow 300ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;border-radius:4px;box-shadow:none;font-size:0.875rem;line-height:1.25rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:rgb(1, 67, 97);background-color:rgb(229, 246, 253);display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;padding:6px 16px;}.css-152gid5 .MuiAlert-icon{color:#03a9f4;}</style><div class="MuiPaper-root MuiPaper-elevation MuiPaper-rounded MuiPaper-elevation0 MuiAlert-root MuiAlert-standardInfo MuiAlert-standard css-152gid5" role="alert"><style data-emotion="css 1l54tgj">.css-1l54tgj{margin-right:12px;padding:7px 0;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;font-size:22px;opacity:0.9;}</style><div class="MuiAlert-icon css-1l54tgj"><style data-emotion="css 1cw4hi4">.css-1cw4hi4{-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;width:1em;height:1em;display:inline-block;fill:currentColor;-webkit-flex-shrink:0;-ms-flex-negative:0;flex-shrink:0;-webkit-transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;transition:fill 200ms cubic-bezier(0.4, 0, 0.2, 1) 0ms;font-size:inherit;}</style><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeInherit css-1cw4hi4" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="InfoOutlinedIcon"><path d="M11,9H13V7H11M12,20C7.59,20 4,16.41 4,12C4,7.59 7.59,4 12,4C16.41,4 20,7.59 20, 12C20,16.41 16.41,20 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10, 10 0 0,0 12,2M11,17H13V11H11V17Z"/></svg></div><style data-emotion="css 1xsto0d">.css-1xsto0d{padding:8px 0;min-width:0;overflow:auto;}</style><div class="MuiAlert-message css-1xsto0d"><style data-emotion="css 1h2y6o4">.css-1h2y6o4{font-weight:500;margin-top:-2px;}</style><style data-emotion="css 17ioyuk">.css-17ioyuk{margin:0;font-size:1rem;line-height:1.5rem;font-family:"Helvetica Neue","sans-serif";font-weight:400;font-style:normal;color:#666666;margin-bottom:0.35em;font-weight:500;margin-top:-2px;}</style><div class="MuiTypography-root MuiTypography-body1 MuiTypography-gutterBottom MuiAlertTitle-root css-17ioyuk">注意</div><p><code>MAX_EXECUTION_TIME</code> 的精度大约为 100ms。在 TiDB 终止 SQL 语句之前，TiKV 中的任务可能已经开始执行。为了减少这种情况下 TiKV 的资源消耗，建议将系统变量 <a href="/zh/tidb/stable/system-variables#tidb_enable_paging-从-v540-版本开始引入"><code>tidb_enable_paging</code></a> 的值设置为 <code>ON</code>。</p></div></div></div><p>删除该 SQL 绑定可以移除限制。</p><pre><code class="language-sql">DROP GLOBAL BINDING for SELECT * FROM t1, t2 WHERE t1.id = t2.id; </code></pre><h2 id="tidb-对哪些-mysql-variables-兼容" style="position:relative"><a href="#tidb-%E5%AF%B9%E5%93%AA%E4%BA%9B-mysql-variables-%E5%85%BC%E5%AE%B9" aria-label="tidb 对哪些 mysql variables 兼容 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>TiDB 对哪些 MySQL variables 兼容？</h2><p>详细可参考<a href="/zh/tidb/stable/system-variables">系统变量</a>。</p><h2 id="省略-order-by-条件时-tidb-中返回结果的顺序与-mysql-中的不一致" style="position:relative"><a href="#%E7%9C%81%E7%95%A5-order-by-%E6%9D%A1%E4%BB%B6%E6%97%B6-tidb-%E4%B8%AD%E8%BF%94%E5%9B%9E%E7%BB%93%E6%9E%9C%E7%9A%84%E9%A1%BA%E5%BA%8F%E4%B8%8E-mysql-%E4%B8%AD%E7%9A%84%E4%B8%8D%E4%B8%80%E8%87%B4" aria-label="省略 order by 条件时 tidb 中返回结果的顺序与 mysql 中的不一致 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>省略 <code>ORDER BY</code> 条件时 TiDB 中返回结果的顺序与 MySQL 中的不一致</h2><p>这不是 bug。返回结果的顺序视不同情况而定，不保证顺序统一。</p><p>MySQL 中，返回结果的顺序可能较为固定，因为查询是通过单线程执行的。但升级到新版本后，查询计划也可能变化。无论是否期待返回结果有序，都推荐使用 <code>ORDER BY</code> 条件。</p><p><a href="http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt" target="_blank" referrerpolicy="no-referrer-when-downgrade">ISO/IEC 9075:1992, Database Language SQL- July 30, 1992</a> 对此有如下表述：</p><blockquote><p>If an <code>&lt;order by clause&gt;</code> is not specified, then the table specified by the <code>&lt;cursor specification&gt;</code> is T and the ordering of rows in T is implementation-dependent.（如果未指定 <code>&lt;order by 条件&gt;</code>，通过 <code>&lt;cursor specification&gt;</code> 指定的表为 T，那么 T 表中的行顺序视执行情况而定。）</p></blockquote><p>以下两条查询的结果都是合法的：</p><pre><code class="language-sql">&gt; select * from t; +------+------+ | a | b | +------+------+ | 1 | 1 | | 2 | 2 | +------+------+ 2 rows in set (0.00 sec) </code></pre><pre><code class="language-sql">&gt; select * from t; -- 不确定返回结果的顺序 +------+------+ | a | b | +------+------+ | 2 | 2 | | 1 | 1 | +------+------+ 2 rows in set (0.00 sec) </code></pre><p>如果 <code>ORDER BY</code> 中使用的列不是唯一列，就无法确定该语句返回结果的顺序。在以下示例中，<code>a</code> 列有重复值，因此只有 <code>ORDER BY a, b</code> 能确定返回结果的顺序。</p><pre><code class="language-sql">&gt; select * from t order by a; +------+------+ | a | b | +------+------+ | 1 | 1 | | 2 | 1 | | 2 | 2 | +------+------+ 3 rows in set (0.00 sec) </code></pre><p>在以下示例中，<code>order by a</code> 能确定 a 列的顺序，但不能确定 b 列的顺序。</p><pre><code class="language-sql">&gt; select * from t order by a; +------+------+ | a | b | +------+------+ | 1 | 1 | | 2 | 2 | | 2 | 1 | +------+------+ 3 rows in set (0.00 sec) </code></pre><p>在 TiDB 中，你还可以使用系统变量 <a href="/zh/tidb/stable/system-variables#tidb_enable_ordered_result_mode"><code>tidb_enable_ordered_result_mode</code></a> 来指定是否对最终的输出结果进行自动排序。</p><h2 id="tidb-是否支持-select-for-update" style="position:relative"><a href="#tidb-%E6%98%AF%E5%90%A6%E6%94%AF%E6%8C%81-select-for-update" aria-label="tidb 是否支持 select for update 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>TiDB 是否支持 <code>SELECT FOR UPDATE</code>？</h2><p>支持。当 TiDB 使用悲观锁（自 TiDB v3.0.8 起默认使用）时，TiDB 中 <code>SELECT FOR UPDATE</code> 的行为与 MySQL 中的基本一致。</p><p>当 TiDB 使用乐观锁时，<code>SELECT FOR UPDATE</code> 不会在事务启动时对数据加锁，而是在提交事务时检查冲突。如果检查出冲突，会回滚待提交的事务。</p><p>详情参考 <a href="/zh/tidb/stable/sql-statement-select#语法元素说明">SELECT 语句语法元素说明</a>。</p><h2 id="tidb-的-codec-能保证-utf8-的字符串是-memcomparable-的吗我们的-key-需要支持-utf8有什么编码建议吗" style="position:relative"><a href="#tidb-%E7%9A%84-codec-%E8%83%BD%E4%BF%9D%E8%AF%81-utf8-%E7%9A%84%E5%AD%97%E7%AC%A6%E4%B8%B2%E6%98%AF-memcomparable-%E7%9A%84%E5%90%97%E6%88%91%E4%BB%AC%E7%9A%84-key-%E9%9C%80%E8%A6%81%E6%94%AF%E6%8C%81-utf8%E6%9C%89%E4%BB%80%E4%B9%88%E7%BC%96%E7%A0%81%E5%BB%BA%E8%AE%AE%E5%90%97" aria-label="tidb 的 codec 能保证 utf8 的字符串是 memcomparable 的吗我们的 key 需要支持 utf8有什么编码建议吗 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>TiDB 的 codec 能保证 UTF8 的字符串是 memcomparable 的吗？我们的 key 需要支持 UTF8，有什么编码建议吗？</h2><p>TiDB 字符集默认就是 UTF8 而且目前只支持 UTF8，字符串就是 memcomparable 格式的。</p><h2 id="一个事务中的语句数量最大是多少" style="position:relative"><a href="#%E4%B8%80%E4%B8%AA%E4%BA%8B%E5%8A%A1%E4%B8%AD%E7%9A%84%E8%AF%AD%E5%8F%A5%E6%95%B0%E9%87%8F%E6%9C%80%E5%A4%A7%E6%98%AF%E5%A4%9A%E5%B0%91" aria-label="一个事务中的语句数量最大是多少 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>一个事务中的语句数量最大是多少？</h2><p>一个事务中的语句数量，默认限制最大为 5000 条。</p><p>在使用乐观事务并开启事务重试的情况下，默认限制 5000，可通过 <a href="/zh/tidb/stable/tidb-configuration-file#stmt-count-limit"><code>stmt-count-limit</code></a> 调整。</p><h2 id="tidb-中为什么出现后插入数据的自增-id-反而小" style="position:relative"><a href="#tidb-%E4%B8%AD%E4%B8%BA%E4%BB%80%E4%B9%88%E5%87%BA%E7%8E%B0%E5%90%8E%E6%8F%92%E5%85%A5%E6%95%B0%E6%8D%AE%E7%9A%84%E8%87%AA%E5%A2%9E-id-%E5%8F%8D%E8%80%8C%E5%B0%8F" aria-label="tidb 中为什么出现后插入数据的自增 id 反而小 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>TiDB 中，为什么出现后插入数据的自增 ID 反而小？</h2><p>TiDB 的自增 ID (<code>AUTO_INCREMENT</code>) 只保证自增且唯一，并不保证连续分配。TiDB 目前采用批量分配的方式，所以如果在多台 TiDB server 上同时插入数据，分配的自增 ID 会不连续。当多个线程并发往不同的 TiDB server 插入数据的时候，有可能会出现后插入的数据自增 ID 小的情况。此外，TiDB 允许给整型类型的字段指定 AUTO_INCREMENT，且一个表只允许一个属性为 <code>AUTO_INCREMENT</code> 的字段。详情可参考<a href="/zh/tidb/stable/mysql-compatibility#自增-id">自增 ID</a>和 <a href="/zh/tidb/stable/auto-increment">AUTO_INCREMENT</a>。</p><h2 id="如何在-tidb-中修改-sql_mode" style="position:relative"><a href="#%E5%A6%82%E4%BD%95%E5%9C%A8-tidb-%E4%B8%AD%E4%BF%AE%E6%94%B9-sql_mode" aria-label="如何在 tidb 中修改 sql_mode 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>如何在 TiDB 中修改 <code>sql_mode</code>？</h2><p>TiDB 支持在会话或全局作用域上修改 <a href="/zh/tidb/stable/system-variables#sql_mode"><code>sql_mode</code></a> 系统变量。</p><ul><li>对全局作用域变量的修改，设置后将作用于集群中的其它服务器，并且重启后更改依然有效。因此，你无需在每台 TiDB 服务器上都更改 <code>sql_mode</code> 的值。</li><li>对会话作用域变量的修改，设置后只影响当前会话，重启后更改消失。</li></ul><h2 id="用-sqoop-批量写入-tidb-数据虽然配置了---batch-选项但还是会遇到-javasqlbatchupdateexecptionstatement-count-5001-exceeds-the-transaction-limitation-的错误该如何解决" style="position:relative"><a href="#%E7%94%A8-sqoop-%E6%89%B9%E9%87%8F%E5%86%99%E5%85%A5-tidb-%E6%95%B0%E6%8D%AE%E8%99%BD%E7%84%B6%E9%85%8D%E7%BD%AE%E4%BA%86---batch-%E9%80%89%E9%A1%B9%E4%BD%86%E8%BF%98%E6%98%AF%E4%BC%9A%E9%81%87%E5%88%B0-javasqlbatchupdateexecptionstatement-count-5001-exceeds-the-transaction-limitation-%E7%9A%84%E9%94%99%E8%AF%AF%E8%AF%A5%E5%A6%82%E4%BD%95%E8%A7%A3%E5%86%B3" aria-label="用 sqoop 批量写入 tidb 数据虽然配置了 batch 选项但还是会遇到 javasqlbatchupdateexecptionstatement count 5001 exceeds the transaction limitation 的错误该如何解决 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 Sqoop 批量写入 TiDB 数据，虽然配置了 <code>--batch</code> 选项，但还是会遇到 <code>java.sql.BatchUpdateExecption:statement count 5001 exceeds the transaction limitation</code> 的错误，该如何解决？</h2><p>问题原因：在 Sqoop 中，<code>--batch</code> 是指每个批次提交 100 条 statement，但是默认每个 statement 包含 100 条 SQL 语句，所以此时 100 * 100 = 10000 条 SQL 语句，超出了 TiDB 的事务限制 5000 条。</p><p>解决办法：</p><ul><li><p>增加选项 <code>-Dsqoop.export.records.per.statement=10</code>，完整的用法如下：</p><pre><code class="language-bash">sqoop export \ -Dsqoop.export.records.per.statement=10 \ --connect jdbc:mysql://mysql.example.com/sqoop \ --username sqoop ${user} \ --password ${passwd} \ --table ${tab_name} \ --export-dir ${dir} \ --batch </code></pre></li><li><p>也可以选择增大 TiDB 的单个事物语句数量限制，不过此操作会导致内存增加。详情参见 <a href="/zh/tidb/stable/tidb-limitations#sql-statements-的限制">SQL 语句的限制</a>。</p></li></ul><h2 id="tidb-有像-oracle-那样的-flashback-query-功能么ddl-支持么" style="position:relative"><a href="#tidb-%E6%9C%89%E5%83%8F-oracle-%E9%82%A3%E6%A0%B7%E7%9A%84-flashback-query-%E5%8A%9F%E8%83%BD%E4%B9%88ddl-%E6%94%AF%E6%8C%81%E4%B9%88" aria-label="tidb 有像 oracle 那样的 flashback query 功能么ddl 支持么 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>TiDB 有像 Oracle 那样的 Flashback Query 功能么，DDL 支持么？</h2><p>有，也支持 DDL。详细参考<a href="/zh/tidb/stable/as-of-timestamp">使用 AS OF TIMESTAMP 语法读取历史数据</a>。</p><h2 id="tidb-中删除数据后会立即释放空间吗" style="position:relative"><a href="#tidb-%E4%B8%AD%E5%88%A0%E9%99%A4%E6%95%B0%E6%8D%AE%E5%90%8E%E4%BC%9A%E7%AB%8B%E5%8D%B3%E9%87%8A%E6%94%BE%E7%A9%BA%E9%97%B4%E5%90%97" aria-label="tidb 中删除数据后会立即释放空间吗 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>TiDB 中删除数据后会立即释放空间吗？</h2><p>在 TiDB 中使用 <code>DELETE</code>，<code>TRUNCATE</code> 和 <code>DROP</code> 语句删除数据都不会立即释放空间。对于 <code>TRUNCATE</code> 和 <code>DROP</code> 操作，在达到 TiDB 的 GC (garbage collection) 时间后（默认 10 分钟），TiDB 的 GC 机制会删除数据并释放空间。对于 DELETE 操作，TiDB 的 GC 机制会删除数据，但不会立即释放空间，而是等到后续进行 compaction 时释放空间。</p><h2 id="删除数据后查询速度为何会变慢" style="position:relative"><a href="#%E5%88%A0%E9%99%A4%E6%95%B0%E6%8D%AE%E5%90%8E%E6%9F%A5%E8%AF%A2%E9%80%9F%E5%BA%A6%E4%B8%BA%E4%BD%95%E4%BC%9A%E5%8F%98%E6%85%A2" aria-label="删除数据后查询速度为何会变慢 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>删除数据后查询速度为何会变慢？</h2><p>删除大量数据后，会有很多无用的 key 存在，影响查询效率。要解决该问题，可以尝试开启 <a href="/zh/tidb/stable/massive-regions-best-practices#方法五开启-region-merge">Region Merge</a> 功能，具体可参考<a href="https://pingcap.com/blog-cn/tidb-best-practice/" target="_blank" referrerpolicy="no-referrer-when-downgrade">最佳实践</a>中的删除数据部分。</p><h2 id="对数据做删除操作之后空间回收比较慢如何处理" style="position:relative"><a href="#%E5%AF%B9%E6%95%B0%E6%8D%AE%E5%81%9A%E5%88%A0%E9%99%A4%E6%93%8D%E4%BD%9C%E4%B9%8B%E5%90%8E%E7%A9%BA%E9%97%B4%E5%9B%9E%E6%94%B6%E6%AF%94%E8%BE%83%E6%85%A2%E5%A6%82%E4%BD%95%E5%A4%84%E7%90%86" aria-label="对数据做删除操作之后空间回收比较慢如何处理 permalink" class="anchor before"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewbox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"/></svg></a>对数据做删除操作之后，空间回收比较慢，如何处理？</h2><p>TiDB 采用了多版本并发控制 (MVCC) 机制，当新写入的数据覆盖旧的数据时，旧的数据不会被替换掉，而是与新写入的数据同时保留，并以时间戳来区分版本。为了使并发事务能查看到早期版本的数据，删除数据时 TiDB 不会立即回收空间，而是等待一段时间后再进行垃圾回收 (GC)。要配置历史数据的保留时限，你可以修改系统变量 <a href="/zh/tidb/stable/system-variables#tidb_gc_life_time-从-v50-版本开始引入"><code>tidb_gc_life_time</code></a>的值（默认值为 <code>10m0s</code>）。</p><h2 id="show-processlist-是否显示系统进程号" style="position:relative"><a href="#show-processlist-%E6%98%AF%E5%90%A6%E6%98%BE%E7%A4%BA%E7%B3%BB%E7%BB%9F%E8%BF%9B%E7%A8%8B%E5%8F%B7" aria-label="show processlist 是否显示系统进程号 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><code>SHOW PROCESSLIST</code> 是否显示系统进程号？</h2><p>TiDB 中的 <code>SHOW PROCESSLIST</code> 与 MySQL 中的 <code>SHOW PROCESSLIST</code> 显示内容基本一致，不会显示系统进程号。而返回结果中的 ID 表示当前的 session ID。其中 TiDB 的 <code>SHOW PROCESSLIST</code> 和 MySQL 的 <code>SHOW PROCESSLIST</code> 区别如下：</p><ul><li><p>由于 TiDB 是分布式数据库，TiDB server 实例是无状态的 SQL 解析和执行引擎（详情可参考 <a href="/zh/tidb/stable/tidb-architecture">TiDB 整体架构</a>），用户使用 MySQL 客户端登录的是哪个 TiDB server，<code>SHOW PROCESSLIST</code> 就会显示当前连接的这个 TiDB server 中执行的 session 列表，不是整个集群中运行的全部 session 列表；而 MySQL 是单机数据库，<code>SHOW PROCESSLIST</code> 列出的是当前整个 MySQL 数据库的全部执行 SQL 列表。</p></li><li><p>在查询执行期间，TiDB 中的 <code>State</code> 列不会持续更新。由于 TiDB 支持并行查询，每个语句可能同时处于多个状态，因此很难显示为某一种状态。</p></li></ul><h2 id="在-tidb-中如何控制或改变-sql-提交的执行优先级" style="position:relative"><a href="#%E5%9C%A8-tidb-%E4%B8%AD%E5%A6%82%E4%BD%95%E6%8E%A7%E5%88%B6%E6%88%96%E6%94%B9%E5%8F%98-sql-%E6%8F%90%E4%BA%A4%E7%9A%84%E6%89%A7%E8%A1%8C%E4%BC%98%E5%85%88%E7%BA%A7" aria-label="在 tidb 中如何控制或改变 sql 提交的执行优先级 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>在 TiDB 中如何控制或改变 SQL 提交的执行优先级？</h2><p>TiDB 支持改变<a href="/zh/tidb/stable/system-variables#tidb_force_priority">全局</a>或单个语句的优先级。优先级包括：</p><ul><li><code>HIGH_PRIORITY</code>：该语句为高优先级语句，TiDB 在执行阶段会优先处理这条语句</li><li><code>LOW_PRIORITY</code>：该语句为低优先级语句，TiDB 在执行阶段会降低这条语句的优先级</li><li><code>DELAYED</code>：该语句为正常优先级语句，TiDB 不强制改变这条语句的优先级，与 <code>tidb_force_priority</code> 设置为 <code>NO_PRIORITY</code> 相同</li></ul><p>以上两种参数可以结合 TiDB 的 DML 语言进行使用，使用方法举例如下：</p><ol><li><p>通过在数据库中写 SQL 的方式来调整优先级：</p><pre><code class="language-sql">SELECT HIGH_PRIORITY | LOW_PRIORITY | DELAYED COUNT(*) FROM table_name; INSERT HIGH_PRIORITY | LOW_PRIORITY | DELAYED INTO table_name insert_values; DELETE HIGH_PRIORITY | LOW_PRIORITY | DELAYED FROM table_name; UPDATE HIGH_PRIORITY | LOW_PRIORITY | DELAYED table_reference SET assignment_list WHERE where_condition; REPLACE HIGH_PRIORITY | LOW_PRIORITY | DELAYED INTO table_name; </code></pre></li><li><p>全表扫会自动调整为低优先级，<a href="/zh/tidb/stable/sql-statement-analyze-table"><code>ANALYZE</code></a> 也是默认低优先级。</p></li></ol><h2 id="在-tidb-中-auto-analyze-的触发策略是怎样的" style="position:relative"><a href="#%E5%9C%A8-tidb-%E4%B8%AD-auto-analyze-%E7%9A%84%E8%A7%A6%E5%8F%91%E7%AD%96%E7%95%A5%E6%98%AF%E6%80%8E%E6%A0%B7%E7%9A%84" aria-label="在 tidb 中 auto analyze 的触发策略是怎样的 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>在 TiDB 中 auto analyze 的触发策略是怎样的？</h2><p>触发策略：如果一张新表达到 1000 条记录，并且在 1 分钟内没有写入，会自动触发。</p><p>当表的（修改数/当前总行数）比例大于 <code>tidb_auto_analyze_ratio</code> 的时候，会自动触发 <code>analyze</code> 语句。<code>tidb_auto_analyze_ratio</code> 的默认值为 0.5，即默认开启触发 auto analyze。为了保险起见，在开启 auto analyze 的时候，<code>tidb_auto_analyze_ratio</code> 的最小值为 0.3。但是该变量值不能大于等于 <code>pseudo-estimate-ratio</code>（默认值为 0.8），否则会有一段时间使用 pseudo 统计信息，建议设置值为 0.5。</p><p>你可以用系统变量 <a href="/zh/tidb/stable/system-variables#tidb_enable_auto_analyze-从-v610-版本开始引入"><code>tidb_enable_auto_analyze</code></a> 关闭 auto analyze。</p><h2 id="可以使用-optimizer-hints-控制优化器行为吗" style="position:relative"><a href="#%E5%8F%AF%E4%BB%A5%E4%BD%BF%E7%94%A8-optimizer-hints-%E6%8E%A7%E5%88%B6%E4%BC%98%E5%8C%96%E5%99%A8%E8%A1%8C%E4%B8%BA%E5%90%97" aria-label="可以使用 optimizer hints 控制优化器行为吗 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>可以使用 Optimizer Hints 控制优化器行为吗？</h2><p>在 TiDB 中，你可以用多种方法控制查询优化器的默认行为，包括使用 <a href="/zh/tidb/stable/optimizer-hints">Optimizer Hints</a> 和 <a href="/zh/tidb/stable/sql-plan-management">SQL 执行计划管理 (SPM)</a>。基本用法同 MySQL 中的一致，还包含若干 TiDB 特有的用法，例如：<code>select column_name from table_name use index（index_name）where where_condition;</code>。</p><h2 id="ddl-执行" style="position:relative"><a href="#ddl-%E6%89%A7%E8%A1%8C" aria-label="ddl 执行 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>DDL 执行</h2><p>本节列出了 DDL 语句执行的相关问题。DDL 执行原理的详细说明，参见 <a href="/zh/tidb/stable/ddl-introduction">TiDB 中 DDL 执行原理及最佳实践</a>。</p><h3 id="各类-ddl-操作的预估耗时是多长" style="position:relative"><a href="#%E5%90%84%E7%B1%BB-ddl-%E6%93%8D%E4%BD%9C%E7%9A%84%E9%A2%84%E4%BC%B0%E8%80%97%E6%97%B6%E6%98%AF%E5%A4%9A%E9%95%BF" aria-label="各类 ddl 操作的预估耗时是多长 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>各类 DDL 操作的预估耗时是多长？</h3><p>假设 DDL 操作没有被阻塞，各个 TiDB server 能够正常更新 Schema 版本，DDL Owner 节点正常运行。在此情况下，各类 DDL 操作的预估耗时如下：</p><table><thead><tr><th align="left">DDL 操作类型</th><th align="left">预估耗时</th></tr></thead><tbody><tr><td align="left">Reorg DDL：例如 <code>ADD INDEX</code>、<code>MODIFY COLUMN</code>（Reorg 类型的数据更改）</td><td align="left">取决于数据量、系统负载以及 DDL 参数的设置</td></tr><tr><td align="left">General DDL（除 Reorg DDL 外的 DDL 类型）：例如 <code>CREATE DATABASE</code>、<code>CREATE TABLE</code>、<code>DROP DATABASE</code>、<code>DROP TABLE</code>、<code>TRUNCATE TABLE</code>、<code>ALTER TABLE ADD</code>、<code>ALTER TABLE DROP</code>、<code>MODIFY COLUMN</code>（只更改元数据）、<code>DROP INDEX</code></td><td align="left">1 秒左右</td></tr></tbody></table><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>以上为各类操作的预估耗时，请以实际操作耗时为准。</p></div></div></div><h2 id="执行-ddl-会慢的可能原因" style="position:relative"><a href="#%E6%89%A7%E8%A1%8C-ddl-%E4%BC%9A%E6%85%A2%E7%9A%84%E5%8F%AF%E8%83%BD%E5%8E%9F%E5%9B%A0" aria-label="执行 ddl 会慢的可能原因 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>执行 DDL 会慢的可能原因</h2><ul><li><p>在一个用户会话中，DDL 语句之前有非 auto-commit 的 DML 语句，并且该 DML 语句的提交操作比较慢，会导致 DDL 语句执行慢。即执行 DDL 语句前，会先提交之前没有提交的 DML 语句。</p></li><li><p>多个 DDL 语句一起执行的时候，后面的几个 DDL 语句可能会比较慢，因为可能需要排队等待。排队场景包括：</p><ul><li>同一类型 DDL 语句需要排队（例如 <code>CREATE TABLE</code> 和 <code>CREATE DATABASE</code> 都是 General DDL，两个操作同时执行时，需要排队）。自 TiDB v6.2.0 起，支持并行 DDL 语句，但为了避免 DDL 使用过多 TiDB 的计算资源，也有并发度限制，因此会有一定的排队情况。</li><li>对同一张表上执行的 DDL 操作存在依赖关系，后面的 DDL 语句需要等待前面的 DDL 操作完成。</li></ul></li><li><p>在集群正常启动后，第一个 DDL 操作的执行时间可能会比较久，可能是因为 DDL 模块在进行 DDL Owner 的选举。</p></li><li><p>终止 TiDB 时，TiDB 不能与 PD 正常通信（包括停电的情况），或者用 <code>kill -9</code> 命令终止 TiDB 导致 TiDB 没有及时从 PD 清理注册数据。</p></li><li><p>集群中某个 TiDB 与 PD 或者 TiKV 之间发生通信问题，即 TiDB 不能及时获取最新版本信息。</p></li></ul><h3 id="触发-information-schema-is-changed-错误的原因" style="position:relative"><a href="#%E8%A7%A6%E5%8F%91-information-schema-is-changed-%E9%94%99%E8%AF%AF%E7%9A%84%E5%8E%9F%E5%9B%A0" aria-label="触发 information schema is changed 错误的原因 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>触发 Information schema is changed 错误的原因？</h3><p>TiDB 在执行 SQL 语句时，会根据隔离级别确定一个对象的 <code>schema</code> 版本来处理该 SQL 语句，而且 TiDB 支持在线异步变更 DDL。那么，在执行 DML 的时候可能有 DDL 语句也在执行，而你需要确保每个 SQL 语句在同一个 <code>schema</code> 上执行。所以当执行 DML 时，如果遇到正在执行中的 DDL 操作，TiDB 可能会报 <code>Information schema is changed</code> 的错误。</p><p>从 v6.4.0 开始，TiDB 实现了<a href="/zh/tidb/stable/metadata-lock">元数据锁机制</a>，可以让 DML 语句的执行和 DDL Schema 变更协同进行，可以避免大部分 <code>Information schema is changed</code> 错误的发生。</p><p>报错的可能原因如下：</p><ul><li>原因 1：正在执行的 DML 所涉及的表和集群中正在执行的 DDL 的表有相同的，那么这个 DML 语句就会报此错。可以通过命令 <code>admin show ddl job</code> 查看正在执行的 DDL 操作。</li><li>原因 2：这个 DML 执行时间很久，而这段时间内执行了很多 DDL 语句，导致中间 <code>schema</code> 版本变更次数超过 1024 （此为默认值，可以通过 <code>tidb_max_delta_schema_count</code> 变量修改）。</li><li>原因 3：接受 DML 请求的 TiDB 长时间不能加载到 <code>schema information</code>（TiDB 与 PD 或 TiKV 之间的网络连接故障等会导致此问题），而这段时间内执行了很多 DDL 语句，导致中间 <code>schema</code> 版本变更次数超过 100。</li><li>原因 4：TiDB 重启后执行第一个 DDL 操作前，执行 DML 操作，并且在执行过程中遇到了第 1 个 DDL 操作（即在执行第 1 个 DDL 操作前，启动该 DML 对应的事务，且在该 DDL 变更第一个 <code>schema</code> 版本后，提交该 DML 对应的事务），那么这个 DML 会报此错。</li></ul><p>以上原因中，只有原因 1 与表有关。原因 1 和原因 2 都不会导致业务问题，相应的 DML 会在失败后重试。对于原因 3，需要检查 TiDB 实例和 PD 及 TiKV 的网络情况。</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><ul><li>目前 TiDB 未缓存所有的 <code>schema</code> 版本信息。</li><li>对于每个 DDL 操作，<code>schema</code> 版本变更的数量与对应 <code>schema state</code> 变更的次数一致。</li><li>不同的 DDL 操作版本变更次数不一样。例如，<code>create table</code> 操作会有 1 次 <code>schema</code> 版本变更；<code>add column</code> 操作有 4 次 <code>schema</code> 版本变更。</li></ul></div></div></div><h3 id="触发-information-schema-is-out-of-date-错误的原因" style="position:relative"><a href="#%E8%A7%A6%E5%8F%91-information-schema-is-out-of-date-%E9%94%99%E8%AF%AF%E7%9A%84%E5%8E%9F%E5%9B%A0" aria-label="触发 information schema is out of date 错误的原因 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>触发 Information schema is out of date 错误的原因？</h3><p>在 v6.5.0 之前，当执行 DML 时，TiDB 超过一个 DDL lease 时间（默认 45s）没能加载到最新的 schema 就可能会报 <code>Information schema is out of date</code> 的错误。遇到此错的可能原因如下：</p><ul><li>执行此 DML 的 TiDB 被 kill 后准备退出，且此 DML 对应的事务执行时间超过一个 DDL lease，在事务提交时会报这个错误。</li><li>TiDB 在执行此 DML 时，有一段时间内连不上 PD 或者 TiKV，导致 TiDB 超过一个 DDL lease 时间没有 load schema，或者导致 TiDB 断开与 PD 之间带 keep alive 设置的连接。</li></ul><h3 id="高并发情况下执行-ddl-时报错的原因" style="position:relative"><a href="#%E9%AB%98%E5%B9%B6%E5%8F%91%E6%83%85%E5%86%B5%E4%B8%8B%E6%89%A7%E8%A1%8C-ddl-%E6%97%B6%E6%8A%A5%E9%94%99%E7%9A%84%E5%8E%9F%E5%9B%A0" aria-label="高并发情况下执行 ddl 时报错的原因 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>高并发情况下执行 DDL 时报错的原因？</h3><p>高并发场景下执行 DDL 语句（比如批量建表）时，极少部分的 DDL 语句可能会由于并发执行时 key 冲突而执行失败。</p><p>并发执行 DDL 语句时，建议将 DDL 语句数量保持在 20 以下，否则你需要在应用端重试失败的 DDL 语句。</p><h3 id="ddl-执行被阻塞的原因" style="position:relative"><a href="#ddl-%E6%89%A7%E8%A1%8C%E8%A2%AB%E9%98%BB%E5%A1%9E%E7%9A%84%E5%8E%9F%E5%9B%A0" aria-label="ddl 执行被阻塞的原因 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>DDL 执行被阻塞的原因</h3><p>在 TiDB v6.2.0 前，TiDB 按照 DDL 语句类型将 DDL 分配到两个先入先出的队列中，即 Reorg DDL 进入 Reorg 队列中，General DDL 进入 general 队列中。由于先入先出以及同一张表上的 DDL 语句需要串行执行，多个 DDL 语句在执行过程中可能会出现阻塞的问题。</p><p>例如对于以下 DDL 语句：</p><ul><li>DDL 1：<code>CREATE INDEX idx on t(a int);</code></li><li>DDL 2：<code>ALTER TABLE t ADD COLUMN b int;</code></li><li>DDL 3：<code>CREATE TABLE t1(a int);</code></li></ul><p>由于队列先入先出的限制，DDL 3 需要等待 DDL 2 执行。同时又因为同一张表上的 DDL 语句需要串行执行，DDL 2 需要等待 DDL 1 执行。因此，DDL 3 需要等待 DDL 1 先执行完，即使它们操作在不同的表上。</p><p>在 TiDB v6.2.0 及之后的版本中，TiDB DDL 模块采用了并发框架。在并发的框架下，不再有同一个队列先进先出的问题，而是从所有 DDL 任务中选出可以执行的 DDL 来执行。并且对 Reorg worker 的数量进行了扩充，大概为节点 <code>CPU/4</code>，这使得在并发框架中 TiDB 可以同时为多张表建索引。</p><p>无论是新集群还是从旧版本升级的集群，在 TiDB v6.2 及以上版本中，TiDB 都会自动使用并发框架，用户无需进行调整。</p><h3 id="定位-ddl-执行卡住的问题" style="position:relative"><a href="#%E5%AE%9A%E4%BD%8D-ddl-%E6%89%A7%E8%A1%8C%E5%8D%A1%E4%BD%8F%E7%9A%84%E9%97%AE%E9%A2%98" aria-label="定位 ddl 执行卡住的问题 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>定位 DDL 执行卡住的问题</h3><ol><li>先排除 DDL 语句通常执行慢的可能原因。</li><li>使用以下任一方法找出 DDL owner 节点：<ul><li>通过 <code>curl http://{TiDBIP}:10080/info/all</code> 获取当前集群的 Owner</li><li>通过监控 <strong>DDL</strong> &gt; <strong>DDL META OPM</strong> 查看某个时间段的 Owner</li></ul></li></ol><ul><li>如果 Owner 不存在，尝试手动触发 Owner 选举：<code>curl -X POST http://{TiDBIP}:10080/ddl/owner/resign</code>。</li><li>如果 Owner 存在，导出 Goroutine 堆栈并检查可能卡住的地方。</li></ul><h2 id="sql-优化" style="position:relative"><a href="#sql-%E4%BC%98%E5%8C%96" aria-label="sql 优化 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>SQL 优化</h2><h3 id="tidb-执行计划解读" style="position:relative"><a href="#tidb-%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92%E8%A7%A3%E8%AF%BB" aria-label="tidb 执行计划解读 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>TiDB 执行计划解读</h3><p>详细解读<a href="/zh/tidb/stable/explain-overview">理解 TiDB 执行计划</a>。</p><h3 id="统计信息收集" style="position:relative"><a href="#%E7%BB%9F%E8%AE%A1%E4%BF%A1%E6%81%AF%E6%94%B6%E9%9B%86" aria-label="统计信息收集 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>统计信息收集</h3><p>详细解读<a href="/zh/tidb/stable/statistics">统计信息</a>。</p><h3 id="count-如何加速" style="position:relative"><a href="#count-%E5%A6%82%E4%BD%95%E5%8A%A0%E9%80%9F" aria-label="count 如何加速 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>Count 如何加速？</h3><p>Count 就是暴力扫表，提高并发度能显著提升扫表速度。如要调整并发度，可以使用 <code>tidb_distsql_scan_concurrency</code> 变量，但调整并发度需要同时考虑 CPU 和 I/O 资源。TiDB 每次执行查询时，都要访问 TiKV。在数据量小的情况下，MySQL 的数据都在内存里，而 TiDB 还需要进行一次网络访问。</p><p>加速建议：</p><ul><li>提升硬件配置，可以参考<a href="/zh/tidb/stable/hardware-and-software-requirements">部署建议</a>。</li><li>提升并发度，默认是 10，可以尝试提升到 50，但是一般提升幅度在 2-4 倍之间。</li><li>测试大数据量的 count。</li><li>调优 TiKV 配置，可以参考<a href="/zh/tidb/stable/tune-tikv-memory-performance">性能调优</a>。</li><li>参考<a href="/zh/tidb/stable/coprocessor-cache">下推计算结果缓存</a>。</li></ul><h3 id="查看当前-ddl-的进度" style="position:relative"><a href="#%E6%9F%A5%E7%9C%8B%E5%BD%93%E5%89%8D-ddl-%E7%9A%84%E8%BF%9B%E5%BA%A6" aria-label="查看当前 ddl 的进度 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>查看当前 DDL 的进度？</h3><p>通过 <code>ADMIN SHOW DDL</code> 语句查看当前 job 进度。操作如下：</p><pre><code class="language-sql">ADMIN SHOW DDL;