executor: fix unexpected NotNullFlag in case when expr ret type (#23102)

[ti-srebot]

cherry-pick #23102 to release-5.0-rc
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23340

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---

Signed-off-by: guo-shaoge [email protected]

What problem does this PR solve?

Issue Number: close #23036

Problem Summary:

drop table if exists t1;
create table t1(c1 int not null);
insert into t1 values(1);
select (case when null then c1 end) is null from t1;

Output should be empty set, but TiDB output "1". Because when we fold constant for IsNull expr, optimizer found the result type of CaseWhen expr has NotNullFlag, which is unexpected. So rewriter return false directly

What is changed and how it works?

What's Changed:

1. Erase NotNullFlag from the result type of CaseWhen expr. Because we don't know will any when clause be true until runtime.
2. make types.setTypeFlag() public. (types.setTypeFlag() -> types.SetTypeFlag())

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Integration test: Add cases in expression.TestBuiltin()

Side effects: no

Release note

• fix unexpected NotNullFlag in case when expr ret type

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

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