planner: allow partial indexes for index join

[winoros]

What problem does this PR solve?

Issue Number: close #69524

Problem Summary:

IndexJoin rejected every partial index because partial indexes were treated as undetermined paths. As a result, even when the partial-index predicate was already proven by pushed-down join conditions, IndexJoin could not choose that index.

What changed and how does it work?

This PR allows IndexJoin path construction to consider partial indexes after partial-index predicate checking/pruning has already run. MV indexes remain rejected.

The added tests cover:

• ordinary partial-index rejection when the predicate is not implied by the join condition
• IndexJoin using a safe a IS NOT NULL partial index for inner join and left outer join
• NULL-safe equality (<=>) not using the partial index
• prepared plan cache correctness with partial index and IndexJoin

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Commands:

make bazel_prepare
./tools/check/failpoint-go-test.sh pkg/planner/core/casetest/index -run TestPartialIndexWithPlanCache -count=1
./run-tests.sh -r planner/core/casetest/index/partialindex
make lint
git diff --check

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix an issue that IndexJoin cannot choose a usable partial index.

Summary by CodeRabbit

• Bug Fixes
  • Improved join planning for queries using partial indexes, including better handling of IndexJoin with prepared statements and cached plans.
  • Fixed planner behavior for cases where partial-index conditions are not proven by the join predicate, while still allowing valid non-partial index use.
  • Added coverage for regular, left, and NULL-safe joins to ensure correct plan selection and query results.

[ti-chi-bot]

Skipping CI for Draft Pull Request.
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[ti-chi-bot]

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Details

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• pkg/planner/OWNERS

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR narrows AccessPath.IsIndexJoinUnapplicable() to reject only MV indexes instead of all "undetermined" paths, enabling partial indexes to be used as IndexJoin inner paths when the join predicate proves the partial-index condition. Tests and fixtures were updated accordingly.

Changes

Partial Index IndexJoin Applicability

Layer / File(s)	Summary
Narrowed IsIndexJoinUnapplicable predicate pkg/planner/util/path.go, pkg/planner/core/exhaust_physical_plans.go	IsIndexJoinUnapplicable() now returns true only for non-table paths with a non-nil MV index, replacing the broader IsUndetermined() check; related comments in exhaust_physical_plans.go updated with no functional change.
Integration test coverage for partial-index IndexJoin tests/integrationtest/t/planner/core/casetest/index/partialindex.test, tests/integrationtest/r/.../partialindex.result	New plan_tree-based tests verify IndexJoin rejects partial indexes when predicates aren't proven, and allows them for issue 69524 scenarios (inner/left join, = and <=>), with updated expected plan/result fixtures.
Plan cache regression test pkg/planner/core/casetest/index/index_test.go	Adds a prepared-statement scenario with INL_JOIN over a partial-index inner table, verifying cached plan uses IndexJoin/idx_a and @@last_plan_from_cache equals 1.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Possibly related PRs

• pingcap/tidb#68832: Introduced the partial-index support that this PR further refines by adjusting AccessPath.IsIndexJoinUnapplicable() and related tests.

Suggested labels: approved, lgtm

Suggested reviewers: YangKeao, yudongusa

Poem

A partial index, once shunned and shy,
Now hops the IndexJoin, no longer passed by,
With IS NOT NULL proven true,
The rabbit joins tables, old meets new,
🐇 Hop, hop, plan_tree — the query's in view!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address #69524 by allowing proven partial indexes in IndexJoin, covering the positive outer-join case and the negative <=> case.
Out of Scope Changes check	✅ Passed	The code and tests stay within the stated planner fix and regression coverage, with no clear unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: enabling partial indexes for IndexJoin.
Description check	✅ Passed	The description covers the problem, fix, tests, issue reference, and release note required by the template.

✨ Finishing Touches

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.2219%. Comparing base (d5729fb) to head (3dc02cb).
⚠️ Report is 29 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #69604        +/-   ##
================================================
- Coverage   76.3258%   74.2219%   -2.1040%     
================================================
  Files          2041       2049         +8     
  Lines        561051     580361     +19310     
================================================
+ Hits         428227     430755      +2528     
- Misses       131923     148881     +16958     
+ Partials        901        725       -176     

Flag	Coverage Δ
integration	41.0505% <100.0000%> (+1.4224%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	60.4610% <ø> (ø)
parser	∅ <ø> (∅)
br	47.2083% <ø> (-15.5428%)	⬇️

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[ti-chi-bot]

@winoros: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-integration-realcluster-test-next-gen	3dc02cb	link	true	/test pull-integration-realcluster-test-next-gen

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