[Patch] Adding CORRESPONDING/CORRESPONDING BY to set operation

Hi,

I wrote a patch for adding CORRESPONDING/CORRESPONDING BY to set operation.
It is a task in the todo list. This is how the patch works:

I modified transformSetOperationStmt() to get an intersection target list
which is the intersection of the target lists of the left clause and right
clause for a set operation statement (sostmt). The intersection target list
is calculated in transformSetOperationTree() and then I modified the target
lists of the larg and rarg of sostmt to make them equal to the intersection
target list. Also, I also changed the target list in pstate->p_rtable in
order to make it consistent with the intersection target list.

I attached the scratch version of this patch to the email. I am not sure
whether the method used in the patch is acceptable or not, but any
suggestions are appreciated. I will add tests and other related things to
the patch if the method used in this patch is acceptable.

Best,

Ruijia