Performance regression found for BenchmarkStringIndexScan

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

http://www.zenlife.tk:18081/alloc

Apparently, yet another regression!

2022-02-07~2022-02-08

Found by #25754

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

16cd0e2

[tiancaiamao]

cd session;
go test -run XXX -bench  BenchmarkStringIndexScan -benchtime 30s -benchmem -memprofile mem.out

#32016 is highly suspected @wjhuang2016

master
BenchmarkStringIndexScan-16 135544 263787 ns/op 97440 B/op 1681 allocs/op

pr32016
BenchmarkStringIndexScan-16 142130 251971 ns/op 94065 B/op 1478 allocs/op

[tiancaiamao]

From @wjhuang2016, after enabling collation, this regression is expected.
So close the issue.