planner: fix a estimation case when index.hist.NDV is greater than index.topN.num.

[AilinKid]

What problem does this PR solve?

Issue Number: close https://github.com/pingcap/tidb/issues/61792

Problem Summary:

What changed and how does it work?

Check List

Tests

• [ ] Unit test
• [x] 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

[tiprow[bot]]

Hi @AilinKid. Thanks for your PR.

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 73.8472%. Comparing base (8310982) to head (83df8b1). :warning: Report is 585 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61793        +/-   ##
================================================
+ Coverage   73.0493%   73.8472%   +0.7978%     
================================================
  Files          1730       1736         +6     
  Lines        481618     487972      +6354     
================================================
+ Hits         351819     360354      +8535     
+ Misses       108261     106019      -2242     
- Partials      21538      21599        +61     

Flag	Coverage Δ
integration	42.6503% <70.0000%> (?)
unit	72.6877% <70.0000%> (+0.4042%)	:arrow_up:

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Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	46.6145% <ø> (+0.0286%)	:arrow_up:

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[hawkingrei]

@AilinKid Please remember to update the test result sets.

[AilinKid]

/test check-dev2

[tiprow[bot]]

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[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-06-18 04:09:04.068581476 +0000 UTC m=+244796.791760458: :ballot_box_with_check: agreed by hawkingrei.
• 2025-06-20 06:56:49.769921258 +0000 UTC m=+427662.493100239: :ballot_box_with_check: agreed by qw4990.

[tiprow[bot]]

@AilinKid: 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
fast_test_tiprow	83df8b1e6df93eca67833565d9e989e55bd5d675	link	true	/test fast_test_tiprow

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[AilinKid]

/test check-dev2

[tiprow[bot]]

@AilinKid: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/test check-dev2

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[terry1purcell]

-1

[terry1purcell]

Added a negative 1 comment above for the following reasons:

1. Questions were raised by two reviewers about the need for "isFullLoad" check in this PR - but neither comment was answered/resolved in this PR.
2. If the usage of isFullLoad is correct - it implies that the accuracy of notNullCount() is of concern if the statistics are not fully loaded. However, if this IF check fails (because of !isFullLoad), then the next step in the code (after that IF) will go ahead and use notNullCount() to calculate avgRowEstimate. That line should also have the same accuracy risk. This PR therefore appears to be incomplete.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #63592. But this PR has conflicts, please resolve them!