docs: document partial indexes for ADD INDEX

[YangKeao]

What is changed, added, or deleted? (Required)

Document partial-index syntax, scenarios, ALTER TABLE ... ADD INDEX examples, predicate behavior, and limitations on the ADD INDEX reference page.

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

• Related code change PR links (if applicable):
• This PR is translated from:
• Other reference link(s): https://github.com/pingcap/docs/blob/master/sql-statements/sql-statement-create-index.md#partial-indexes

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• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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Summary by CodeRabbit

• Documentation
  • Updated the ADD INDEX reference to document partial indexes.
  • Added syntax support for WHERE predicates in index definitions.
  • Included usage scenarios, examples with EXPLAIN output, and documented limitations.

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• sql-statements/sql-statement-add-index.md

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📝 Walkthrough

Walkthrough

The ADD INDEX reference now includes WHERE predicates in its grammar and documents partial indexes, including syntax, examples, EXPLAIN output, supported predicates, and limitations.

Changes

Partial index documentation

Layer / File(s)	Summary
Partial index syntax and reference sql-statements/sql-statement-add-index.md	The IndexOption grammar accepts a WHERE expression. A new section documents partial and partial unique indexes, examples, query plans, supported predicates, type requirements, table references, and expression-index limitations.

Priority: ⬇️ Low

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

Change: Other

Merge Risk: ⚪ Minimal · up to e2927

This documentation-only change is safe to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: documenting partial indexes for ADD INDEX.
Description check	✅ Passed	The description includes the required change summary, affected TiDB versions, related documentation link, AI involvement, and change-category checklist. The first-time contributor section is optional …
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.

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Timeline:

• 2026-09-22 11:14:32.212819774 +0000 UTC m=+103397.438040881: ☑️ agreed by dveeden.