planner: enable fix control 52869 by default to let planner consider IndexMerge when an alternative index exists | tidb-test=pr/2753

[time-and-fate]

Manual cherry-pick of #63701

---

What problem does this PR solve?

Issue Number: ref #26764

Problem Summary:

Please see the issue and the doc.

We have been advising users to enable this flag for similar scenarios, and received no bad cases or bug reports. As we encounter this situation more and more often, we think it makes sense to enable this flag by default.

What changed and how does it work?

Set the default value to ON by default, and update the test results.

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

• Query Optimization
  • Adjusted index-merge and range-scan decision behavior; some queries may now produce different execution plans, changing which indexes drive lookups and predicate placement.
• Tests
  • Updated expected query plan traces and cardinality-estimation debug outputs to reflect the new optimizer behavior.

[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: 9b584322-b2eb-45a3-a3dc-e7958885b9cd

📥 Commits

Reviewing files that changed from the base of the PR and between 59390fb and 0d470d3.

📒 Files selected for processing (2)

• pkg/planner/cardinality/testdata/cardinality_suite_out.json
• tests/integrationtest/r/imdbload.result

✅ Files skipped from review due to trivial changes (1)

• tests/integrationtest/r/imdbload.result

---

📝 Walkthrough

Walkthrough

The PR changes Fix52869 default to true for index-merge range-scan validation and makes skylinePruning set preferMerge solely from explicit IndexMergeHints, causing planner selection and CE trace outputs to change across related tests.

Changes

IndexMerge Path Optimization Behavior

Layer / File(s)	Summary
Optimizer feature flag default and path preference logic pkg/planner/core/indexmerge_path.go, pkg/planner/core/find_best_task.go	generateIndexMerge4NormalIndex now treats fixcontrol.Fix52869 as true by default for skipRangeScanCheck; skylinePruning sets preferMerge based only on len(ds.IndexMergeHints) > 0.
Index access path plan expectations tests/integrationtest/r/planner/core/indexmerge_path.result	Multiple EXPLAIN result expectations updated: equality+OR cases now show IndexLookUp with probe-side selections; several range/OR cases now show IndexMerge unions with secondary index range scans and adjusted TableRowIDScan estimates.
Cardinality/CE trace updates pkg/planner/cardinality/testdata/cardinality_suite_out.json, tests/integrationtest/r/imdbload.result	CE trace JSON entries for OR-range predicates are split into multiple Index Stats-Range items with revised row_count values in both unit and integration test traces.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

size/S, cherry-pick-approved, ok-to-test, type/cherry-pick-for-release-8.5

Suggested reviewers

• qw4990
• terry1purcell
• winoros

Poem

🐰 A tiny hop, a flag set true,
The planner trims the paths anew.
Hints now whisper which way to roam,
Traces split like crumbs back home.
Optimizer hums — the rabbit cheers!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% 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 accurately describes the main change: enabling fix control 52869 by default to allow planner to consider IndexMerge when alternative indexes exist.
Description check	✅ Passed	The PR description follows the template structure and includes all required sections: issue reference, problem summary, what changed, checklist, side effects, and release notes.
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.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[time-and-fate]

/retest

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-8.5@2fce0f9). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #68920   +/-   ##
================================================
  Coverage               ?   55.8546%           
================================================
  Files                  ?       1826           
  Lines                  ?     664246           
  Branches               ?          0           
================================================
  Hits                   ?     371012           
  Misses                 ?     266374           
  Partials               ?      26860           

Flag	Coverage Δ
integration	39.2769% <100.0000%> (?)
unit	65.9364% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	55.3108% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	54.9383% <0.0000%> (?)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[time-and-fate]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

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
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-04 14:40:30.780344609 +0000 UTC m=+452531.850661999: ☑️ agreed by qw4990.
• 2026-06-04 15:37:37.529575824 +0000 UTC m=+455958.599893214: ☑️ agreed by terry1purcell.

[time-and-fate]

/retest