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statistics: Added note about no TopN collected for unique single columns #21633

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statistics: Added note about no TopN collected for unique single columns #21633

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Expand Up @@ -98,6 +98,10 @@ Top-N 值是列或索引中出现次数前 N 的值。Top-N 统计信息通常

TiDB 会记录 Top-N 的值和出现次数。参数 `WITH NUM TOPN` 控制 Top-N 值的数量，默认值是 `20`，表示收集出现频率最高的前 20 个值；最大值是 `1024`。关于该参数的详细信息，参见[手动收集](#手动收集)小节。

> **注意：**
>
> 从 v9.0.0 开始，当 [`tidb_analyze_version = 2`](/system-variables.md#tidb_analyze_version-new-in-v510) 时，TiDB 不会为单列唯一索引及其对应的列收集 Top-N 统计信息，因为每个值最多只会出现一次。
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## 选择性收集统计信息

本小节介绍如何选择性地收集统计信息。
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