expression: Fix wrong charset and collation for case when function

[wshwsh12]

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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[sre-bot]

Please follow PR Title Format:

• pkg [, pkg2, pkg3]: what's changed

Or if the count of mainly changed packages are more than 3, use

• *: what's changed

[wshwsh12]

/run-integration-tests

[wshwsh12]

/run-integration-tests

[wshwsh12]

/run-integration-tests

[XuHuaiyu]

LGTM

[XuHuaiyu]

/merge

[ti-chi-bot]

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

Commit hash: 5a4bf46

[wshwsh12]

/rebuild

[ti-srebot]

cherry pick to release-4.0 in PR #26671

[ti-srebot]

cherry pick to release-5.0 in PR #26672

[ti-srebot]

cherry pick to release-5.1 in PR #26673