executor: union scan refactor, introduce the the mem rows iterator

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #43249

Problem Summary:

This is the optimization of the UnionScan, part1

What is changed and how it works?

In the old code, all the mem buff rows are drained in the unionscan.Open() stage.
The commit try to introduce a memRowsIter data struct, the memRowsIter is built at the Open stage, but it does not fetch data in some cases(Now just handle the table reader and !desc).
Instead, the memRowsIter provide the Next() method for the caller to call it at the Next() stage.

Check List

Tests

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

Change the test case to 'select * from t limit 1'

git diff
diff --git a/executor/union_scan_test.go b/executor/union_scan_test.go
index ecc3e0b6bb..ca4c297a78 100644
--- a/executor/union_scan_test.go
+++ b/executor/union_scan_test.go
@@ -567,7 +567,7 @@ c6 datetime);`)
        b.ReportAllocs()
        b.ResetTimer()
        for i := 0; i < b.N; i++ {
-               tk.MustQuery("select * from t_us where c1 = '12345'").Check(testkit.Rows())
+               tk.MustQuery("select c1 from t_us limit 1").Check(testkit.Rows("54321"))
        }
        b.StopTimer()
 }

go test -run XXX -tags intest -bench BenchmarkUnionScan -cpuprofile cpu1.out -benchtime 45s -benchmem
Before:
12007           4545021 ns/op         2875056 B/op      41671 allocs/op
After:
15372           3704359 ns/op          326602 B/op      10063 allocs/op

And flamegraph before vs after:

• 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]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[hawkingrei]

/test tiprow_fast_test

[ti-chi-bot]

@hawkingrei: The specified target(s) for /test were not found.
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/test tiprow_fast_test

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[tiancaiamao]

/retest

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/retest

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[tiancaiamao]

/retest

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/retest

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcwangchao, xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-27 03:24:25.27314399 +0000 UTC m=+678030.674394441: ☑️ agreed by lcwangchao.
• 2023-06-27 07:52:11.870120593 +0000 UTC m=+694097.271371041: ☑️ agreed by xhebox.

[tiancaiamao]

/merge

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