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statistics: Added note about no TopN collected for unique single columns #22475

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statistics: Added note about no TopN collected for unique single columns
mjonss Feb 19, 2026
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Update statistics.md
mjonss Feb 20, 2026
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Expand Up @@ -110,6 +110,10 @@ Top-N values are values with the top N occurrences in a column or index. Top-N s

TiDB records the values and occurrences of Top-N values. Here `N` is controlled by the `WITH NUM TOPN` parameter. The default value is 20, meaning the top 20 most frequent values are collected. The maximum value is 1024. For details about the parameter, see [Manual collection](#manual-collection).

> **Note:**
>
> When [`tidb_analyze_version = 2`](/system-variables.md#tidb_analyze_version-new-in-v510), TiDB does not collect Top-N statistics for unique indexes that contain only a single column, or for the corresponding column statistics. This is because each value can occur at most once.

## Selective statistics collection

This section describes how to collect statistics selectively.
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