expression: Fix wrong charset and collation for case when function (#26663)

[ti-srebot]

cherry-pick #26663 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26672

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/26672:release-5.0-6421cecb1297

---

What problem does this PR solve?

Issue Number: close #26662

Problem Summary:

Wrong collation will lead expression.ColumnSubstitute can't generate newFilter for LogicalProjection in ppdSolver.
Because there is a collation check in ColumnSubstitute.

tidb/expression/util.go

Lines 234 to 245 in b073d6b

	if collate.NewCollationEnabled() {
	// Make sure the collation used by the ScalarFunction isn't changed and its result collation is not weaker than the collation used by the ScalarFunction.
	if changed {
	changed = false
	tmpArgs := make([]Expression, 0, len(v.GetArgs()))
	_ = append(append(append(tmpArgs, refExprArr.Result()[0:idx]...), refExprArr.Result()[idx+1:]...), newFuncExpr)
	_, newColl := DeriveCollationFromExprs(v.GetCtx(), append(v.GetArgs(), newFuncExpr)...)
	if coll == newColl {
	changed = checkCollationStrictness(coll, newFuncExpr.GetType().Collate)
	}
	}
	}

What is changed and how it works?

What's Changed:
Use function DeriveCollationFromExprs to get collation for caseWhen function

How it Works:

Check List

Tests

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

Fix wrong charset and collation for case when function

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• bb7133

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wshwsh12 you're already a collaborator in bot's repo.

[XuHuaiyu]

LGTM

[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: fb434f3

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