planner: add group ndv risk variables

[Tristan1900]

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• Other reference link(s): planner: use expoBackoff for GROUP BY cardinality estimation when no … tidb#62813

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[gemini-code-assist]

Summary of Changes

Hello @Tristan1900, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces documentation for a new TiDB optimizer variable, tidb_opt_group_ndv_skew_ratio. This variable provides fine-grained control over how the query optimizer estimates the Number of Distinct Values (NDV) for grouped data, allowing users to adjust the balance between different estimation methods. This enhancement aims to offer more flexibility in optimizing query plans, potentially leading to improved performance for specific workloads.

Highlights

• New Optimizer Variable Documentation: Added documentation for tidb_opt_group_ndv_skew_ratio, a new system variable introduced in TiDB v9.0.0. This variable controls the ratio between naive estimation and exponential backoff estimation for group Number of Distinct Values (NDV), allowing for a weighted combination of these methods.

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[Tristan1900]

wait till #21339 get merged

[gemini-code-assist]

Code Review

This pull request adds documentation for the new system variable tidb_opt_group_ndv_skew_ratio. The added content is clear and follows the document's structure. I've suggested a minor wording improvement to make the description more concise and easier to read.

[gemini-code-assist]

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) ↩