planner: improve comments and clean up function for OR type IndexMerge logic

[time-and-fate]

What problem does this PR solve?

Issue Number: ref #58361

What changed and how does it work?

• The argument of the two slice parameter of generateNormalIndexPartialPath() always has 1 length, so I directly changed the parameter type.
• Try to make it more clear and easier to understand by renaming some variable/function names and cleaning up some comments.

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

[codecov]

Codecov Report

Attention: Patch coverage is 83.56164% with 12 lines in your changes missing coverage. Please review.

Project coverage is 73.6659%. Comparing base (8a80a41) to head (a274dbd).
Report is 19 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58332        +/-   ##
================================================
+ Coverage   73.2172%   73.6659%   +0.4486%     
================================================
  Files          1679       1679                
  Lines        462451     462485        +34     
================================================
+ Hits         338594     340694      +2100     
+ Misses       103079     100988      -2091     
- Partials      20778      20803        +25     

Flag	Coverage Δ
integration	43.2509% <79.4520%> (?)
unit	72.3629% <73.9726%> (+0.0073%)	⬆️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	46.0230% <ø> (+0.0120%)	⬆️

[tiprow]

@time-and-fate: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	a274dbd	link	true	/test fast_test_tiprow

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[Rustin170506]

Thanks!

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, Rustin170506

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• pkg/planner/OWNERS [AilinKid,Rustin170506]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-18 07:53:25.757878217 +0000 UTC m=+1029795.846680760: ☑️ agreed by AilinKid.
• 2024-12-18 09:01:13.511182716 +0000 UTC m=+1033863.599985259: ☑️ agreed by Rustin170506.