statistics: add comments and change isIndex to bool

[Rustin170506]

What problem does this PR solve?

Issue Number: None

Problem Summary:

What is changed and how it works?

• Added more comments.
• Change isIndex to bool

Check List

Tests

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

  • Only update comment

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 #47062 (1a70aac) into master (51f6bb9) will decrease coverage by 0.3461%.
The diff coverage is 81.5789%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #47062        +/-   ##
================================================
- Coverage   73.0205%   72.6744%   -0.3461%     
================================================
  Files          1336       1357        +21     
  Lines        398462     404757      +6295     
================================================
+ Hits         290959     294155      +3196     
- Misses        88697      91897      +3200     
+ Partials      18806      18705       -101     

Flag	Coverage Δ
integration	30.4981% <81.5789%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <ø> (ø)
parser	84.9620% <ø> (ø)
br	48.6578% <ø> (-4.3491%)	⬇️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [hawkingrei,winoros]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-18 09:08:43.246979321 +0000 UTC m=+508489.214567372: ☑️ agreed by hawkingrei.
• 2023-09-18 09:14:41.716665712 +0000 UTC m=+508847.684253762: ☑️ agreed by winoros.

[Rustin170506]

/retest