planner: support semi join rewrite as an alternative logical plan

[AilinKid]

What problem does this PR solve?

Issue Number: close #67467

Problem Summary:

The optimizer can use SEMI_JOIN_REWRITE when it is forced by hint or session variable, but alternative logical plan search did not try that rewrite as a competing logical plan when alternative logical plans are enabled.

What changed and how does it work?

This PR records when the first logical plan build sees a semi join that can try SEMI_JOIN_REWRITE, and adds an alternative logical-plan round that temporarily enables semi join rewrite for that build+optimize round. The new round follows the existing alternativeRound setup/cleanup pattern and restores EnableSemiJoinRewrite after the round finishes.

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.

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.

None

Summary by CodeRabbit

Release Notes

• Tests

  • Expanded test coverage for semi-join rewrite operations and updated expected query execution plans for correlated subquery cases.

• Query Optimization

  • Enhanced optimizer to evaluate alternative semi-join rewrite strategies when appropriate configuration flags are enabled, with updated plan selection logic.

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a new "semi-join-rewrite" alternative logical plan round to the optimizer. A new AlternativeLogicalPlanSemiJoinRewrite flag is introduced in StatementContext, set by buildSemiJoin when conditions are met, and consumed by the optimizer loop to temporarily enable EnableSemiJoinRewrite for one extra planning round. Failpoint conditions and test expectations are updated accordingly.

Changes

Semi-join rewrite alternative logical plan round

Layer / File(s)	Summary
StatementContext signal pkg/sessionctx/stmtctx/stmtctx.go	Adds AlternativeLogicalPlanSemiJoinRewrite bool field, resets it in ResetAlternativeLogicalPlanSignals, and adds MarkAlternativeLogicalPlanSemiJoinRewrite() method.
buildSemiJoin marks signal pkg/planner/core/logical_plan_builder.go	In buildSemiJoin, calls MarkAlternativeLogicalPlanSemiJoinRewrite() when alternative plans are enabled, forced rewrite is off, and EnableSemiJoinRewrite is disabled.
Optimizer: new alternative round pkg/planner/optimize.go	Adds shouldTrySemiJoinRewriteRound predicate, savedEnableSemiJoinRewrite for state restore, and a new "semi-join-rewrite" entry in alternativeRounds that temporarily enables EnableSemiJoinRewrite. Updates the failpoint trigger to match round.name+":"+OriginalText().
Failpoint updates and test coverage pkg/planner/core/casetest/correlated/correlated_test.go, pkg/planner/core/casetest/rule/rule_outer_to_semi_join_test.go, pkg/planner/core/casetest/rule/testdata/correlate_suite_out.json, pkg/planner/core/casetest/rule/testdata/correlate_suite_xut.json	Updates correlated test failpoint payloads to use "non-decorrelate:" prefix; extends TestSemiJoinRewrite with an assertion that DELETE ... IN (subquery) uses IndexHashJoin without the hint; updates plan shape expectations in both correlate suite JSON files.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• pingcap/tidb#66206: Implements CorrelateSolver and correlated-subquery test suites whose planning behavior is directly affected by the new semi-join-rewrite alternative round and StatementContext flag.
• pingcap/tidb#68752: Extends the correlate-subquery planning pipeline that this PR's new round feeds into.
• pingcap/tidb#68796: Modifies the same alternative logical-plan infrastructure in pkg/planner/optimize.go and StatementContext that this PR extends with the semi-join-rewrite round.

Suggested labels

ok-to-test

Suggested reviewers

• qw4990
• windtalker
• yudongusa

Poem

🐇 A semi-join stood all alone in the queue,
No hint to rewrite it — what should the planner do?
A new alternative round now checks for the sign,
Temporarily enables the flag — how divine!
The optimizer hops and selects the best plan,
IndexHashJoin chosen as fast as it can! 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding support for semi join rewrite as an alternative logical plan during optimization.
Description check	✅ Passed	The PR description follows the template structure with Issue Number, Problem Summary, What Changed/How It Works, and all required checklists. It adequately explains the motivation and implementation approach.
Linked Issues check	✅ Passed	The PR fully addresses issue #67467 objectives: recording semi joins during logical plan build, adding an alternative round with temporary semi-join rewrite enablement, and including unit tests verifying the functionality.
Out of Scope Changes check	✅ Passed	All changes are in-scope: test updates for semi-join rewrite behavior, failpoint modifications for test correctness, new session context flag for tracking alternative plans, and optimizer logic updates to implement the feature.

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✨ Finishing Touches

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Warning

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🔧 golangci-lint (2.12.2)

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[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.5896%. Comparing base (0b376ea) to head (d3aaafb).
⚠️ Report is 25 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #69279        +/-   ##
================================================
- Coverage   76.3199%   74.5896%   -1.7304%     
================================================
  Files          2041       2034         -7     
  Lines        562128     575119     +12991     
================================================
- Hits         429016     428979        -37     
- Misses       132205     145609     +13404     
+ Partials        907        531       -376     

Flag	Coverage Δ
integration	41.1170% <36.3636%> (+1.4592%)	⬆️

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	47.5536% <ø> (-15.2186%)	⬇️

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[AilinKid]

/test mysql-test

[qw4990]

/retest

[AilinKid]

/retest-required

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qw4990, terry1purcell

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Details

Needs approval from an approver in each of these files:

• OWNERS [qw4990,terry1purcell]
• pkg/planner/OWNERS [qw4990,terry1purcell]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-18 07:42:10.307280525 +0000 UTC m=+1637031.377597905: ☑️ agreed by qw4990.
• 2026-06-20 16:59:40.640987049 +0000 UTC m=+1843281.711304439: ☑️ agreed by terry1purcell.

[qw4990]

/retest

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@pkg/planner/optimize.go`:
- Around line 637-640: The package-level variable `savedEnableSemiJoinRewrite`
creates a shared mutable state that can cause race conditions when multiple
sessions call the optimize function concurrently, as different sessions' cleanup
operations can overwrite each other's saved values. Move the
`savedEnableSemiJoinRewrite` variable from package scope to be local within the
optimize function or the specific invocation scope where it is used, and update
the cleanup code (around lines 678-679) that restores the
`EnableSemiJoinRewrite` value to reference the invocation-local variable instead
of the global one. This ensures each session maintains its own isolated state
during the semi-join-rewrite round.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: ae47a475-38b3-461d-a896-5fddc2804e8c

📥 Commits

Reviewing files that changed from the base of the PR and between 234eee9 and d3aaafb.

📒 Files selected for processing (7)

• pkg/planner/core/casetest/correlated/correlated_test.go
• pkg/planner/core/casetest/rule/rule_outer_to_semi_join_test.go
• pkg/planner/core/casetest/rule/testdata/correlate_suite_out.json
• pkg/planner/core/casetest/rule/testdata/correlate_suite_xut.json
• pkg/planner/core/logical_plan_builder.go
• pkg/planner/optimize.go
• pkg/sessionctx/stmtctx/stmtctx.go

[coderabbitai]

⚠️ Potential issue | 🟠 Major | ⚡ Quick win

Scope EnableSemiJoinRewrite restore state to the current invocation.

Line [637] introduces a package-level mutable savedEnableSemiJoinRewrite; concurrent optimize calls across sessions can race and restore the wrong session value in cleanup (Lines [678-679]). This should be invocation-local, not global state.

Suggested fix

-// savedEnableSemiJoinRewrite holds the pre-round value of
-// EnableSemiJoinRewrite so setup/cleanup can restore it after the
-// semi-join-rewrite round. Safe because optimize is single-threaded per session.
-var savedEnableSemiJoinRewrite bool
-
 var alternativeRounds = []alternativeRound{
@@
 	{
 		name:    "semi-join-rewrite",
 		enabled: shouldTrySemiJoinRewriteRound,
 		setup: func(sv *variable.SessionVars) {
-			savedEnableSemiJoinRewrite = sv.EnableSemiJoinRewrite
 			sv.EnableSemiJoinRewrite = true
 		},
 		cleanup: func(sv *variable.SessionVars) {
-			sv.EnableSemiJoinRewrite = savedEnableSemiJoinRewrite
+			sv.EnableSemiJoinRewrite = false
 		},
 	},

Also applies to: 675-680

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@pkg/planner/optimize.go` around lines 637 - 640, The package-level variable
`savedEnableSemiJoinRewrite` creates a shared mutable state that can cause race
conditions when multiple sessions call the optimize function concurrently, as
different sessions' cleanup operations can overwrite each other's saved values.
Move the `savedEnableSemiJoinRewrite` variable from package scope to be local
within the optimize function or the specific invocation scope where it is used,
and update the cleanup code (around lines 678-679) that restores the
`EnableSemiJoinRewrite` value to reference the invocation-local variable instead
of the global one. This ensures each session maintains its own isolated state
during the semi-join-rewrite round.

[AilinKid]

/test check-dev2