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planner: add group ndv risk variables #21654

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13 changes: 13 additions & 0 deletions system-variables.md
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Expand Up @@ -4861,6 +4861,19 @@ SHOW WARNINGS;

</details>

### tidb_opt_group_ndv_skew_ratio <span class="version-mark">New in v9.0.0</span>

> **Warning:**
>
> The TiDB query optimizer uses this variable. Do not adjust this variable without guidance from TiDB support or engineering.

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Float
- Range: `[0, 1]`
- Default value: `0`
- This variable controls the ratio between naive estimation and exponential backoff estimation for group NDV (Number of Distinct Values). When set to `0`, it uses naive estimation. When set to `1`, it uses exponential backoff estimation. Values between `0` and `1` provide a weighted combination of both estimation methods.
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low

For better readability and conciseness, you can combine the description of this variable into a single sentence. This avoids repeating "estimation" and "When set to".1

Suggested change
- This variable controls the ratio between naive estimation and exponential backoff estimation for group NDV (Number of Distinct Values). When set to `0`, it uses naive estimation. When set to `1`, it uses exponential backoff estimation. Values between `0` and `1` provide a weighted combination of both estimation methods.
- This variable controls the ratio between naive estimation and exponential backoff for group NDV (Number of Distinct Values): `0` for naive estimation, `1` for exponential backoff, and values in between for a weighted combination of both.

Style Guide References

Footnotes

  1. The style guide recommends avoiding unnecessary words and repetition. (link) ↩


### tidb_opt_scan_factor

- Scope: SESSION | GLOBAL
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