statistics: reduce small obj alloc for PreCalculateScalar

[winoros]

What problem does this PR solve?

Issue Number: close #52343

Problem Summary:

We don't need to re-alloc anything since we only read from the chunk, no modification after reading it.

What changed and how does it work?

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

Merging #52623 (5cb7dfa) into master (5aa48d8) will increase coverage by 2.0646%.
Report is 11 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #52623        +/-   ##
================================================
+ Coverage   72.3481%   74.4128%   +2.0646%     
================================================
  Files          1470       1471         +1     
  Lines        427158     428133       +975     
================================================
+ Hits         309041     318586      +9545     
+ Misses        98845      89714      -9131     
- Partials      19272      19833       +561     

Flag	Coverage Δ
integration	49.0384% <100.0000%> (?)
unit	71.1782% <100.0000%> (-0.0622%)	⬇️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	49.7258% <ø> (+8.6257%)	⬆️

[hi-rustin]

I'm not very familiar with these structures.
Is it safe to reuse? Are there other places that use it in this way?
If it is, please add some comments to note that we trying to rescue it to save memory here.

[winoros]

I'm not very familiar with these structures. Is it safe to reuse? Are there other places that use it in this way? If it is, please add some comments to note that we trying to rescue it to save memory here.

You can jump into the func convertDatumToScalar, which calculates a float64 score for the datum. So the datum is read-only before it's destroyed.

[hi-rustin]

/hold

Please add some comments to it.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, hi-rustin

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• pkg/statistics/OWNERS [hawkingrei,hi-rustin]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-15 16:35:12.119623414 +0000 UTC m=+269531.230669860: ☑️ agreed by hawkingrei.
• 2024-04-16 11:35:50.175781818 +0000 UTC m=+337969.286828264: ☑️ agreed by hi-rustin.

[winoros]

/unhold

[winoros]

/cherrypick release-8.1

[ti-chi-bot]

@winoros: new pull request created to branch release-8.1: #52652.

In response to this:

/cherrypick release-8.1

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