executor: do not read from lock cache when snapshot read (#21529)

[ti-srebot]

cherry-pick #21529 to release-4.0

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Signed-off-by: you06 [email protected]

What problem does this PR solve?

Issue Number: close #21447

Problem Summary:

What is changed and how it works?

What's Changed:

When read from snapshot and key not exist in membuffer, do not look up from lock cache.

How it Works:

Lock key will write lock cache, if the key is locked but the value is not affected the locked value will only be written into lock cache, the cached value should not read by a snapshot read.

Related changes

• Need to cherry-pick to the release-4.0

Check List

Tests

• Unit test

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

@you06 you're already a collaborator in bot's repo.

[you06]

/run-all-tests

[cfzjywxk]

Seems batch point get processing is missing?

[you06]

/run-all-tests

[you06]

Seems batch point get processing is missing?

Fixed.

[jackysp]

LGTM

[cfzjywxk]

@jebter I think we need to do some benchmark after merging this PR, maybe there will be some benchmark improvement relying on the mistakenly used pessimistic lock cache, for example like

select * from t where pk = 1 for update ;
select * from t where pk = 1;
select * from t where pk = 1;

There may be some regression for this kind of benchmark.

[cfzjywxk]

LGTM

[you06]

Here's the result testing with https://gist.github.com/you06/ac94678ccfb69448238e31955ebd5350

thread	count	689914f	this PR
1	1024	0.526ms/1896	1.274ms/784
2	1024	0.402ms/4958	1.087ms/1837
4	1024	0.350ms/11399	1.134ms/3521
8	1024	0.353ms/22569	1.008ms/7924
16	1024	0.469ms/34003	1.010ms/15824
32	1024	0.662ms/48259	1.456ms/21955
64	1024	1.073ms/59572	2.079ms/30766
128	1024	1.991ms/64261	3.348ms/38216
256	1024	3.132ms/81699	5.188ms/49333
512	1024	6.483ms/78963	9.363ms/54674

The latency reading from lock cache is much lower, however point get from tikv seems efficient enough.

@cfzjywxk cc