planner: reduce topn count to exclude non-skewed values | tidb-test=pr/2333

[terry1purcell]

What problem does this PR solve?

Issue Number: close #53111

Problem Summary:

What changed and how does it work?

PR includes 3 changes to reduce unnecessary TopN values:

1. Reduce the default TopN to 100 (from 500).
2. Support pruning of TopN values that are "not skewed" for smaller tables.
3. And initially avoid adding values with a count=1 (which will be pruned anyway).

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

[ti-chi-bot]

Hi @terry1purcell. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[hawkingrei]

/ok-to-test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.5456%. Comparing base (2069651) to head (6bde95a).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #53035        +/-   ##
================================================
+ Coverage   72.5215%   74.5456%   +2.0241%     
================================================
  Files          1506       1506                
  Lines        431006     431348       +342     
================================================
+ Hits         312572     321551      +8979     
+ Misses        99055      89861      -9194     
- Partials      19379      19936       +557     

Flag	Coverage Δ
integration	49.1072% <78.5714%> (?)
unit	71.4343% <100.0000%> (-0.0742%)	⬇️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	50.4537% <ø> (+8.6338%)	⬆️

[ti-chi-bot]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ZL91
Once this PR has been reviewed and has the lgtm label, please assign elsa0520 for approval. For more information see the Code Review Process.

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[elsa0520]

Maybe PR shouldn't remove the README.md ?

[elsa0520]

Could you please explain why we changed the TopNNum from 500 to 100 in comments ?

[elsa0520]

Should we add some new test about the pruning logic in smaller table ? And check the topn number which has been pruned ?