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?