planner: Add new variable named "tidb_opt_selectivity_factor" #21655
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Summary of Changes
Hello @elsa0520, 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 aims to document a new system variable. While the title and description indicate the addition of tidb_opt_skew_distinct_agg, the provided patch actually adds documentation for tidb_opt_selectivity_factor. This variable is crucial for the TiDB optimizer, defining a pseudo selectivity value used when statistical information is insufficient.
Highlights
- New System Variable Documentation: Documentation for the tidb_opt_selectivity_factor system variable has been added to system-variables.md. This variable, despite the PR title mentioning tidb_opt_skew_distinct_agg, is described with its scope (SESSION | GLOBAL), persistence, type (Float), range ([0, 1]), and default value (0.8). It indicates the pseudo selectivity for the TiDB optimizer and is generally not recommended for modification.
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Code Review
This pull request aims to add documentation for the tidb_opt_skew_distinct_agg system variable. However, the changes in this pull request add documentation for tidb_opt_selectivity_factor instead. Please check if this is the intended change. The added documentation for tidb_opt_selectivity_factor is mostly correct, but I have a minor suggestion to improve the clarity of the description.
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
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First-time contributors' checklist
What is changed, added or deleted? (Required)
Add new variable named "tidb_opt_skew_distinct_agg" and explain it
Which TiDB version(s) do your changes apply to? (Required)
Tips for choosing the affected version(s):
By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.
For details, see tips for choosing the affected versions.
What is the related PR or file link(s)?
Do your changes match any of the following descriptions?