planner: Add subquery variables

[terry1purcell]

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pingcap/tidb#63204
pingcap/tidb#63416

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[gemini-code-assist]

Summary of Changes

Hello @terry1purcell, 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 updates the system variables documentation to include two new variables, tidb_opt_enable_no_decorrelate_in_select and tidb_opt_enable_semi_join_rewrite. These additions provide users with clearer information on how to manage optimizer hints for subqueries at both session and global levels, enhancing control over query planning behavior.

Highlights

• New System Variable Documentation: Added documentation for the tidb_opt_enable_no_decorrelate_in_select system variable, which controls the application of the NO_DECORRELATE hint for subqueries in the select list.
• New System Variable Documentation: Added documentation for the tidb_opt_enable_semi_join_rewrite system variable, which controls the application of the SEMI_JOIN_REWRITE hint for all queries containing subqueries.

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[gemini-code-assist]

Code Review

This pull request adds documentation for two new system variables: tidb_opt_enable_no_decorrelate_in_select and tidb_opt_enable_semi_join_rewrite. The changes are generally good. I've suggested a few minor improvements for clarity and consistency with the style guide. I also pointed out a potential technical inaccuracy in the description of tidb_opt_enable_semi_join_rewrite based on the existing documentation for the corresponding hint.

[Oreoxmt]

@terry1purcell Could you please invite a tech reviewer? Thanks!

[Oreoxmt]

Rest LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@Oreoxmt: Your lgtm message is repeated, so it is ignored.

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[Oreoxmt]

/cc @qiancai

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-30 01:55:48.248867473 +0000 UTC m=+1528054.326120053: ☑️ agreed by Oreoxmt.
• 2025-11-04 08:09:53.581901089 +0000 UTC m=+171243.024930968: ☑️ agreed by qiancai.