statistics: extract functions and add comments

[Rustin170506]

What problem does this PR solve?

Issue Number: ref #40791

Problem Summary:

What changed and how does it work?

• Extracted some util functions.
• Moved stats_table_row_cache out of cacher inner.
• Add some comments.

No logic change, just some refactors.

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

[Rustin170506]

🔢 Self-check

[codecov]

Codecov Report

Merging #50773 (bd8d342) into master (1a0b7ff) will increase coverage by 2.6610%.
Report is 9 commits behind head on master.
The diff coverage is 88.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #50773        +/-   ##
================================================
+ Coverage   70.4839%   73.1449%   +2.6610%     
================================================
  Files          1463       1464         +1     
  Lines        433198     437408      +4210     
================================================
+ Hits         305335     319942     +14607     
+ Misses       108629      97641     -10988     
- Partials      19234      19825       +591     

Flag	Coverage Δ
integration	49.6997% <87.0000%> (?)
unit	70.8399% <85.0000%> (+0.5648%)	⬆️

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

Components	Coverage Δ
dumpling	54.2940% <ø> (ø)
parser	∅ <ø> (∅)
br	51.2372% <ø> (+5.5227%)	⬆️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-29 04:52:51.592825607 +0000 UTC m=+1368413.157123305: ☑️ agreed by hawkingrei.
• 2024-01-30 02:12:47.937865002 +0000 UTC m=+1445209.502162706: ☑️ agreed by qw4990.