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", appears to be intended to enhance the TiDB documentation by adding details for new system variables related to optimizer selectivity and executor concurrency. However, the provided patch currently only includes the addition of a simple test file named "temp.md" containing "This is a test file.". This suggests the patch might be incomplete or a placeholder for the described documentation changes.

Highlights

• File Addition: A new file, "temp.md", has been added to the repository with the content "This is a test file.".
• Intended Documentation Update (Discrepancy Noted): Although the provided patch only adds a test file, the pull request title and an auto-generated summary indicate an intention to update "system-variables.md". This update would introduce documentation for the "tidb_opt_selectivity_factor" system variable and new per-operator executor concurrency controls (e.g., index lookup, hash join, window functions) for TiDB.

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[gemini-code-assist]

Code Review

This pull request appears to add a temporary file, temp.md, which seems unrelated to the PR's goal of updating system-variables.md. This file should be removed from the commit. The expected changes to system-variables.md are not present in the provided diff.