statistics: fix the fomula for checking outdated stats (#26728)

[ti-srebot]

cherry-pick #26728 to release-5.0
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/26734

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

git push [email protected]:ti-srebot/tidb.git pr/26734:release-5.0-4eeff54d85c6

---

What problem does this PR solve?

Issue Number: close #26282

What is changed and how it works?

Currently, we use (modify count/realtime row count) to check if the stats are outdated. But the reasonable method to calculate the ratio should be (modify count/row count when last analyze).

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• qw4990

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[time-and-fate]

This PR needs to be merged after #26707.

[Reminiscent]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b733655