Oops, missed one fix for EquivalenceClass rearrangement.

Now that we're expecting a mergeclause's left_ec/right_ec to persist from
the initial assignments, we can't just blithely zero these out when
transforming such a clause in adjust_appendrel_attrs.  But really it should
be okay to keep the parent's values, since a child table's derived Var
ought to be equivalent to the parent Var for all EquivalenceClass purposes.
(Indeed, I'm wondering whether we couldn't find a way to dispense with
add_child_rel_equivalences altogether.  But this is wrong in any case.)

src/backend/optimizer/prep/prepunion.c

@@ -1682,13 +1682,13 @@ adjust_appendrel_attrs_mutator(Node *node, AppendRelInfo *context)
 
 		/*
 		 * Reset cached derivative fields, since these might need to have
-		 * different values when considering the child relation.
+		 * different values when considering the child relation.  Note we
+		 * don't reset left_ec/right_ec: each child variable is implicitly
+		 * equivalent to its parent, so still a member of the same EC if any.
 		 */
 		newinfo->eval_cost.startup = -1;
 		newinfo->norm_selec = -1;
 		newinfo->outer_selec = -1;
-		newinfo->left_ec = NULL;
-		newinfo->right_ec = NULL;
 		newinfo->left_em = NULL;
 		newinfo->right_em = NULL;
 		newinfo->scansel_cache = NIL;