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, according to its title and description, aims to update the system-variables.md documentation to reflect new system variables for controlling optimizer selectivity and per-operator executor concurrency in TiDB. However, the provided patch content only shows the addition of a simple test file, temp.md, which contains 'This is a test file.'. This suggests a potential mismatch between the PR's stated purpose and its current file changes.

Highlights

• Intended System Variable Additions: The pull request description indicates the addition of a new system variable, tidb_opt_selectivity_factor, designed to control default optimizer selectivity when statistics are unavailable. It also suggests the introduction of per-operator executor concurrency controls for various operations like index lookup, hash join, and aggregation.
• Intended Documentation Update: The PR's stated purpose, as detailed in the auto-generated summary, is to update the system variables reference documentation with comprehensive details and guidance for the newly introduced optimizer selectivity and executor concurrency settings.
• Actual Changes in Patch: Despite the stated intent to update system-variables.md, the provided patch content only includes the addition of a new file named temp.md, which contains the text 'This is a test file.'.

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[gemini-code-assist]

Code Review

This pull request appears to be incorrect. The title and description suggest an update to the system-variables.md file, but the only change submitted is the addition of temp.md, which seems to be a temporary test file. Please verify that the correct changes have been committed and pushed. The temp.md file should be removed.