Update system-variables.md

[qiancai]

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• This PR is translated from: Update system-variables.md docs-cn#15
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Summary by CodeRabbit

• New Features

  • Added tidb_opt_selectivity_factor system variable (SESSION/GLOBAL, persistent) to control default optimizer selectivity when statistics are unavailable.
  • Introduced per-operator executor concurrency controls (index lookup, index lookup join, hash join, hash aggregation partial/final, projection, window) alongside tidb_executor_concurrency for finer tuning.

• Documentation

  • Updated system variables reference with details and guidance for the new optimizer selectivity and executor concurrency settings.

[gemini-code-assist]

Summary of Changes

Hello @qiancai, 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, titled "Update system-variables.md", primarily introduces a new file named "temp.md" with placeholder content. However, the pull request description and an automated summary suggest a broader purpose: to update the TiDB documentation regarding new system variables. Specifically, it aims to document "tidb_opt_selectivity_factor" for controlling optimizer selectivity and introduce details about per-operator executor concurrency controls. It's important to note that the current patch only reflects the addition of "temp.md", not the documentation updates mentioned.

Highlights

• File Addition: A new file named "temp.md" has been added to the repository, containing the text "This is a test file."
• Intended Documentation Update: The pull request description and an automated summary indicate an intention to update the "system-variables.md" documentation for TiDB.
• New System Variables (Intended): The intended documentation update includes details for a new system variable, "tidb_opt_selectivity_factor", which controls default optimizer selectivity, and new per-operator executor concurrency controls (e.g., for index lookup, hash join, projection, window).
• Discrepancy Noted: There is a discrepancy between the stated intent in the PR description and the actual file changes provided in the patch, which only show the addition of "temp.md" and not the documentation updates.

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[gemini-code-assist]

Code Review

This pull request's title and description indicate an update to system-variables.md, but the only change submitted is the addition of a temp.md file with placeholder content. The intended documentation updates appear to be missing from this pull request. Please push the correct changes to resolve this discrepancy.