statistics: improve topn pruning logic (#34282)

[ti-srebot]

cherry-pick #34282 to release-6.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/34301

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/34301:release-6.0-6bd54bea8a9e

---

What problem does this PR solve?

Issue Number: close #34256 ref #32758

Problem Summary:

Please see the issue.

What is changed and how it works?

1. When the collected TopN's NDV reaches the total NDV, we won't do the pruning now.
2. Fix the wrong otherNDV.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@time-and-fate you're already a collaborator in bot's repo.

[xhebox]

closed for DMR version