planner: flaky test TestIndexNestedLoopHashJoin

[terry1purcell]

What problem does this PR solve?

Issue Number: ref #63501

Problem Summary:

What changed and how does it work?

Reduced flakyness of this test in 2 ways:

1. Claude code's recommendation for resolving the race condition (see below) to "set" the variables using SQL rather than directly assign the variable in the code.
2. Changed the plan comparison to only look for the existence of IndexHashJoin rather than comparing the entire plan.

Claude code's summary of this change:

Why This Fixes (or Reduces) the Race Condition

The race occurs because:

• Test goroutine directly writes to InitChunkSize/MaxChunkSize fields
• Join worker goroutines from ANALYZE or query execution concurrently read these same fields
• No synchronization protects these accesses

Using SET statements helps because:

• MustExec("SET ...") is a blocking call that completes before returning
• Creates a synchronization barrier between variable modification and next query
• Goes through TiDB's proper variable setting infrastructure
• Consistent with rest of test (lines 38-39 already use SET)

Important caveat: This may reduce but not completely eliminate the race if background
goroutines from earlier operations persist and continue reading SessionVars.

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

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[tiprow]

Hi @terry1purcell. Thanks for your PR.

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[Copilot]

Pull request overview

This PR fixes a flaky test TestIndexNestedLoopHashJoin by replacing direct manipulation of session variables with proper SQL SET statements. This ensures thread-safe test execution and prevents race conditions that could cause test instability.

• Replaced direct session variable assignments with SQL SET statements for clustered index and chunk size configurations
• Removed the now-unused vardef package import
• Applied consistent fixes across two test functions: TestIndexNestedLoopHashJoin and TestIssue31129

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.8149%. Comparing base (8aefecd) to head (3e12779).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65012        +/-   ##
================================================
- Coverage   70.9290%   68.8149%   -2.1141%     
================================================
  Files          1888       1866        -22     
  Lines        515954     511005      -4949     
================================================
- Hits         365961     351648     -14313     
- Misses       125582     137029     +11447     
+ Partials      24411      22328      -2083     

Flag	Coverage Δ
integration	41.6473% <ø> (-6.5072%)	⬇️
unit	65.9769% <ø> (+0.3128%)	⬆️

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

Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	39.1496% <ø> (-20.2754%)	⬇️

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[purelind]

/retest

[tiprow]

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[terry1purcell]

/ok-to-test

[ti-chi-bot]

@terry1purcell: 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
idc-jenkins-ci-tidb/mysql-test	3e12779	link	true	/test mysql-test

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