docs: add tidb_dml_max_execution_time system variable- #21929

[gengliqi]

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Add tidb_dml_max_execution_time system variable.

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Summary by CodeRabbit

• Documentation
  • Added documentation for the tidb_dml_max_execution_time system variable.
  • Documented its supported scopes, millisecond-based values, default unlimited setting, and SET_VAR support.
  • Clarified which transactional DML and commit operations are subject to the timeout, along with applicable exclusions and autocommit behavior.
  • Added cross-references in the system-variable reference and SQL timeout guide.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 9a8524d7-58d5-4e08-9473-b76dcaecfd2b

📥 Commits

Reviewing files that changed from the base of the PR and between 49d3949 and e9f3899.

📒 Files selected for processing (3)

• develop/dev-guide-timeouts-in-tidb.md
• system-variable-reference.md
• system-variables.md

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

This change documents tidb_dml_max_execution_time, including its scope, default value, supported statements, exclusions, timeout behavior, and availability. It also adds cross-references in the system-variable reference and TiDB timeout guide.

Changes

DML timeout documentation

Layer / File(s)	Summary
Define and cross-reference DML timeout variable system-variables.md, develop/dev-guide-timeouts-in-tidb.md, system-variable-reference.md	Documents tidb_dml_max_execution_time, including its scopes, range, default, supported statements, transaction behavior, exclusions, and timeout details. Adds links from the system-variable reference and timeout guide.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: qiancai, lilin90, ti-chi-bot

Merge Risk: ⚪ Minimal · up to e9f38

This documentation-only change adds the DML timeout variable and cross-references without altering runtime behavior. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation change for the tidb_dml_max_execution_time system variable. The issue number is acceptable, although the separator before it is slightly awkward.
Description check	✅ Passed	The description includes the required change summary, applicable versions, contributor checklist, AI involvement, and change-category selections. Optional reference-link fields are left blank, which d…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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