this case will enter `if (IsA(groupexpr, RelabelType))` branch.
We set "C" as groupkey's collation, and it's not equal to column's.
In transformGroupClause(), syntax `COLLATE xxx` will create CollateExpr node, and CollateExpr's arg is Var node.
And then, planner will call eval_const_expressions_mutator() to transform CollateExpr to RelableType if CollateExpr->collOid not equal to Collate->arg's collation.
Yech. Thanks for the explanation.
Because exprCollation() return the resultcollid if the nodetag is Relabletype, the expr() && PartKeyCollMatchesExprColl() can handle this situation.
So we don't need "if (IsA(groupexpr, RelableTyple) " this branch.
About V3 patch, PartCollMatchesExprColl() may be not suitable, because collation of partkey must be equal to groupkey's, even though they are all InvalidOid.
I'm not sure about what you said. I uesd it because partition prune do this way. I keep it temporarily.
The test case in v3 patch only has EXPLAIN statement. I add SELECT statement and make the result more stable according to Aleksander advices.
