statistics: reduce memory usage when to MergePartTopN2GlobalTopN (#45718)

[ti-chi-bot]

This is an automated cherry-pick of #45718

What problem does this PR solve?

Issue Number: close #45727

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

add some gobench for it.

Before:

Benchmark	Iterations	Time per Operation	Memory Allocated	Allocations per Operation
BenchmarkMergePartTopN2GlobalTopNWithHists100-8	85184	13737 ns/op	4408 B/op	53 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists1000-8	8985	136583 ns/op	40696 B/op	503 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists10000-8	702	1637701 ns/op	405880 B/op	5003 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists100000-8	48	23604707 ns/op	4000376 B/op	50003 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists1000000-8	5	235231317 ns/op	40002699 B/op	500003 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists10000000-8	1	17470526833 ns/op	400001144 B/op	5000003 allocs/op

After: #45718

Benchmark	Iterations	Time per Operation	Memory Allocated	Allocations per Operation
BenchmarkMergePartTopN2GlobalTopNWithHists100-8	103918	11497 ns/op	120 B/op	2 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists1000-8	10000	113324 ns/op	120 B/op	2 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists10000-8	909	1308099 ns/op	120 B/op	2 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists100000-8	63	17689263 ns/op	120 B/op	2 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists1000000-8	6	180973042 ns/op	120 B/op	2 allocs/op
BenchmarkMergePartTopN2GlobalTopNWithHists10000000-8	1	8101635583 ns/op	120 B/op	2 allocs/op

• No code

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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hi-rustin, time-and-fate

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-10 10:36:51.662186694 +0000 UTC m=+195376.211202680: ☑️ agreed by time-and-fate.
• 2023-08-11 14:01:25.893193718 +0000 UTC m=+294050.442209705: ☑️ agreed by hi-rustin.

[hawkingrei]

/retest

[codecov]

Codecov Report

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The diff coverage is n/a.

Additional details and impacted files

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