Update system-variables.md

[qiancai]

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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.

[coderabbitai]

Walkthrough

Documentation updates in system-variables.md introduce a new optimizer selectivity variable and expand executor concurrency settings with per-operator variables. The selectivity variable appears twice in the document. No code changes are included.

Changes

Cohort / File(s)	Summary of Changes
Optimizer selectivity variable system-variables.md	Added tidb_opt_selectivity_factor (SESSION
Executor per-operator concurrency system-variables.md	Documented per-operator concurrency variables alongside tidb_executor_concurrency: tidb_index_lookup_concurrency, tidb_index_lookup_join_concurrency, tidb_hash_join_concurrency, tidb_hashagg_partial_concurrency, tidb_hashagg_final_concurrency, tidb_projection_concurrency, tidb_window_concurrency.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A rabbit taps docs with careful flair,
New knobs for speed, selections fair.
Concurrency carrots in tidy rows,
While twin selectivity twice now shows.
I thump my foot—change merged with cheer,
Hops per second feel snappier here! 🥕✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Description Check	⚠️ Warning	The pull request description remains as the unfilled template and does not include any of the required information, such as a summary of changes, the TiDB versions affected, related PR or file links, or confirmation of signing the Contributor License Agreement. Because all mandatory sections are left blank, the description does not meet the repository’s template requirements for documentation contributions.	Please complete the description template by providing a clear “What is changed, added or deleted?” section explaining the updates, selecting the appropriate TiDB version checkboxes, adding any related PR or file links, and confirming that you have signed the Contributor License Agreement.
Title Check	❓ Inconclusive	The title “Update system-variables.md” identifies the file that was changed but fails to communicate the substantive updates in this pull request, such as introducing the tidb_opt_selectivity_factor variable and adding per-operator concurrency settings. Its generic phrasing does not highlight the key features or intent of the changes, making it difficult for readers to quickly grasp the purpose of the PR. Because it merely states that the file was updated without specifying what was added or why, the title is too vague to serve as an effective summary.	Please choose a more descriptive title that succinctly reflects the main changes, for example “Add tidb_opt_selectivity_factor and per-operator concurrency variables to system-variables.md,” so that reviewers and future readers can immediately understand the PR’s purpose and content.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changes. Docstring coverage check skipped.

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Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 1566e06 and ba2a185.

📒 Files selected for processing (1)

• system-variables.md (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)

• GitHub Check: pull