statistics: correct the bucket order when merging global hist

[winoros]

What problem does this PR solve?

Issue Number: close #59274

Problem Summary:

What changed and how does it work?

the mergeBuffer is in reverse order. we need to call slices.Reverse before join it with others.

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 72.72727% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.6109%. Comparing base (9b8296b) to head (61976c8).
Report is 14 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59647        +/-   ##
================================================
+ Coverage   72.9702%   73.6109%   +0.6406%     
================================================
  Files          1694       1729        +35     
  Lines        468610     480408     +11798     
================================================
+ Hits         341946     353633     +11687     
+ Misses       105603     105034       -569     
- Partials      21061      21741       +680     

Flag	Coverage Δ
integration	45.8122% <4.5454%> (?)
unit	72.3130% <72.7272%> (+0.1337%)	⬆️

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	44.9537% <ø> (-0.1500%)	⬇️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [AilinKid,hawkingrei]
• pkg/statistics/OWNERS [AilinKid,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-19 09:45:49.558243537 +0000 UTC m=+1040991.954465596: ☑️ agreed by AilinKid.
• 2025-02-19 16:55:30.501761089 +0000 UTC m=+1066772.897983151: ☑️ agreed by hawkingrei.

[hawkingrei]

/retest

[winoros]

the bench function BenchmarkMergePartitionHist2GlobalHist can test it.