planner: fix TPCDS Q64 regression under the new join algorithm

[qw4990]

What problem does this PR solve?

Issue Number: ref #69600

Problem Summary: planner: fix TPCDS Q64 regression under the new CD-C join algorithm

What changed and how does it work?

In TPCDS Q64, ib1 and ib2 have the same row count, but the old join-order implementation is using SortStableFunc while the new one is using SortFunc, which leads to this join order change.

This PR update the new join-order implementation to also use SortStableFunc to keep the prior behavior.

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

• Bug Fixes
  • Improved stability in join-order planning by preserving the relative order of candidate nodes when they have equal estimated cost, avoiding inconsistent tie-breaking.
• Tests
  • Updated TPC-DS Q64 expected plan outputs to reflect revised physical plan structure under exchanges and hash join subtree nesting.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 329b67b1-17a0-4f88-9aa8-cbf41456ef26

📥 Commits

Reviewing files that changed from the base of the PR and between b4e21c2 and 478d198.

📒 Files selected for processing (2)

• pkg/planner/core/casetest/tpcds/testdata/tpcds_suite_out.json
• pkg/planner/core/casetest/tpcds/testdata/tpcds_suite_xut.json

---

📝 Walkthrough

Walkthrough

The greedy join-order optimizer now uses a stable sort for equal-cost candidates, and the TPCDS Q64 expected plan outputs were updated to match the resulting join and exchange ordering.

Changes

Join Order Sort Stability and Q64 Plan Expectations

Layer / File(s)	Summary
Stable sort for node ordering pkg/planner/core/joinorder/join_order.go	joinOrderGreedy.optimize now uses slices.SortStableFunc instead of slices.SortFunc when ordering nodes by cumCost.
Updated Q64 expected plan trees pkg/planner/core/casetest/tpcds/testdata/tpcds_suite_out.json, pkg/planner/core/casetest/tpcds/testdata/tpcds_suite_xut.json	The stored TestTPCDSQ64 plan outputs are rewritten to reflect the new exchange, projection, and hash-join subtree layout.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related issues

• planner: TPCDS Q64 plan regression caused by new cd-c join algorithm #69600: Matches the join-order behavior change and the associated TPCDS Q64 plan output updates.

Possibly related PRs

• pingcap/tidb#68340: Modifies joinOrderGreedy.optimize in the same join-order area and interacts with the candidate-node ordering logic.

Suggested labels: approved, lgtm

Suggested reviewers: AilinKid, windtalker

Poem

I sorted the nodes, then gave them a spin,
But ties stayed in step from the first one to win.
🐇 The Q64 plans now hop in place,
With stable little footprints all over the trace.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: fixing the TPCDS Q64 regression under the new join algorithm.
Description check	✅ Passed	The description matches the template well, including problem summary, issue reference, changes, test checklist, and release note.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.1110%. Comparing base (8be4bd0) to head (478d198).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #69637        +/-   ##
================================================
- Coverage   76.3268%   74.1110%   -2.2159%     
================================================
  Files          2041       2050         +9     
  Lines        560589     576524     +15935     
================================================
- Hits         427880     427268       -612     
- Misses       131808     149049     +17241     
+ Partials        901        207       -694     

Flag	Coverage Δ
integration	40.7041% <100.0000%> (+0.9989%)	⬆️

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

Components	Coverage Δ
dumpling	60.4471% <ø> (ø)
parser	∅ <ø> (∅)
br	47.4650% <ø> (-15.2563%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, winoros

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Details

Needs approval from an approver in each of these files:

• pkg/planner/OWNERS [AilinKid,winoros]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-07-03 08:59:16.98660315 +0000 UTC m=+375498.686982562: ☑️ agreed by winoros.
• 2026-07-03 09:13:22.624763627 +0000 UTC m=+376344.325143040: ☑️ agreed by AilinKid.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #69654.