Update system-variables.md

[qiancai]

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What is the related PR or file link(s)?

• This PR is translated from: Update system-variables.md docs-cn#15
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Do your changes match any of the following descriptions?

• Delete files
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Summary by CodeRabbit

• Documentation
  • Added documentation for a new system variable: tidb_opt_selectivity_factor.
  • Describes purpose (optimizer default selectivity when statistics are insufficient), default value (0.8), valid range ([0, 1]), and scopes (SESSION and GLOBAL).
  • Notes it can be adjusted via SET_VAR but is generally not recommended to change.
  • Placed alongside related selectivity settings for easier discoverability.

[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 for updating the documentation related to TiDB system variables, specifically mentioning the addition of tidb_opt_selectivity_factor and per-operator executor concurrency controls. However, the actual changes provided in the patch only include the addition of a new file, temp.md, containing simple test content. This suggests a potential mismatch between the PR's stated intent and its current patch content.

Highlights

• File Addition: A new file named temp.md has been added to the repository.
• Content of temp.md: The temp.md file contains the simple text 'This is a test file.'.
• Stated Purpose Discrepancy: While the pull request title and description indicate an update to system-variables.md and related feature documentation, the provided patch only shows the addition of temp.md.

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[gemini-code-assist]

Code Review

There appears to be a significant discrepancy in this pull request. The title, description, and auto-generated summary all indicate that the system-variables.md file should be updated. However, the only change submitted is the addition of a placeholder file named temp.md. Please review the pull request to ensure it includes the intended changes. For the file that was submitted, I have included one minor suggestion.

[github-actions]

🤖 Auto-sync completed successfully

📥 Source PR: qiancai/docs-cn#15
🎯 Target PR: #47
✅ English documentation has been updated based on Chinese documentation changes.

This comment was generated automatically by the sync workflow.

[coderabbitai]

Walkthrough

Adds documentation for a new system variable tidb_opt_selectivity_factor (SESSION | GLOBAL, SET_VAR-enabled, float in [0,1], default 0.8) describing its role in optimizer selectivity fallback. The same variable block appears twice in nearby sections, creating a duplicate entry.

Changes

Cohort / File(s)	Summary
Docs — System variables system-variables.md	Added documentation for tidb_opt_selectivity_factor (scope SESSION/GLOBAL, SET_VAR, default 0.8, range [0,1]) describing its use when selectivity cannot be derived from stats; introduced a duplicated block of the same variable in a nearby section.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I nibble notes on fields so fair,
A factor set with careful care—
When stats go shy, we guess the share,
But oops, I see it listed there… and there!
I twitch my whiskers, fix the pair,
Then hop along with tidy flair. 🐇✨

Pre-merge checks and finishing touches

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Title Check	⚠️ Warning	The title “Update system-variables.md” is overly generic and only names the file being modified without indicating the substantive change, which is the introduction of the new tidb_opt_selectivity_factor system variable; it does not clearly convey the primary purpose or scope of the update.	Please revise the title to succinctly describe the main change, for example “Add tidb_opt_selectivity_factor system variable to system-variables.md,” so that reviewers immediately understand the key update.
Description Check	⚠️ Warning	The description includes the standard template structure but leaves critical sections blank—no details are provided for “What is changed, added or deleted,” and the version checklist is entirely unchecked—making it impossible to understand the nature and scope of the changes.	Please fill in the “What is changed, added or deleted?” section with a summary of the new tidb_opt_selectivity_factor variable, mark the applicable TiDB version(s) in the checklist, and confirm any related PR links or CLA status.

✅ Passed checks (1 passed)

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

✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch test-add-a-variable-15

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

system-variables.md (1)

1796-1801: Align metadata labels with existing variable templates

Other entries in this reference use “Persists to cluster”, “Applies to hint [SET_VAR]”, “Type: Float”, and “Range” for these bullets. Please update this block to match the established wording so we stay consistent across the page.

- - Is persisted to the cluster: Yes
- - Is controlled by the Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- - Type: Floating-point number
- - Value range: `[0, 1]`
+ - Persists to cluster: Yes
+ - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
+ - Type: Float
+ - Range: `[0, 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 322c671 and 351e82b.

📒 Files selected for processing (1)

• system-variables.md (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

**/*.md

⚙️ CodeRabbit configuration file

As a technical writer in the database area, you need to first learn the changes in this PR, and then improve the content to make it clear, concise, and easy to understand in English according to your database knowledge.

Files:

• system-variables.md