Re: Properly pathify the union planner

On Wed, Mar 27, 2024 at 6:23 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> Because this field is set, it plans the CTE thinking it's a UNION
> child and breaks when it can't find a SortGroupClause for the CTE's
> target list item.

Right. The problem here is that we mistakenly think that the CTE query
is a subquery for the set operation and thus store the SetOperationStmt
in its qp_extra. Currently the code for the check is:

/*
* Check if we're a subquery for a set operation. If we are, store
* the SetOperationStmt in qp_extra.
*/
if (root->parent_root != NULL &&
root->parent_root->parse->setOperations != NULL &&
IsA(root->parent_root->parse->setOperations, SetOperationStmt))
qp_extra.setop =
(SetOperationStmt *) root->parent_root->parse->setOperations;
else
qp_extra.setop = NULL;

This check cannot tell if the subquery is for a set operation or a CTE,
because its parent might have setOperations set in both cases. Hmm, is
there any way to differentiate between the two?

Thanks
Richard