tidb: add the join order related variable tidb_opt_join_reorder_through_sel

[hfxsd]

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• This PR is translated from:tidb: add the join order related variable tidb_opt_join_reorder_through_sel docs-cn#21292
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[gemini-code-assist]

Summary of Changes

Hello @hfxsd, 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 system variable, tidb_opt_join_reorder_through_sel. This variable is designed to improve join reorder optimization in multi-table JOIN queries by allowing the optimizer to evaluate Selection conditions more effectively. The documentation details its scope, persistence, type, default value, and its impact on execution plans, including recommendations for its use and conditions under which it might be skipped.

Highlights

• New System Variable Documentation: Added comprehensive documentation for tidb_opt_join_reorder_through_sel, a new system variable introduced in TiDB v8.5.6 and v9.0.0, which enhances join reorder optimization.

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Changelog

• system-variables.md
  • Added documentation for the new system variable tidb_opt_join_reorder_through_sel.
  • Included details on its scope, persistence, type, default value, and optimization behavior.
  • Provided guidance on potential performance impacts and non-deterministic filter handling.

Activity

• No specific activity (comments, reviews, progress updates) was provided in the context.

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[gemini-code-assist]

Code Review

The pull request adds documentation for the new system variable tidb_opt_join_reorder_through_sel. The description is clear and technically accurate. I have provided a few minor suggestions to improve consistency with the rest of the document and adhere to the style guide, specifically regarding the use of backticks for operator names and variable values, and the use of active voice.

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