Compare collations before merging UNION operations.

In the dim past we figured it was okay to ignore collations
when combining UNION set-operation nodes into a single N-way
UNION operation.  I believe that was fine at the time, but
it stopped being fine when we added nondeterministic collations:
the semantics of distinct-ness are affected by those.  v17 made
it even less fine by allowing per-child sorting operations to
be merged via MergeAppend, although I think we accidentally
avoided any live bug from that.

Add a check that collations match before deciding that two
UNION nodes are equivalent.  I also failed to resist the
temptation to comment plan_union_children() a little better.

Back-patch to all supported branches (v13 now), since they
all have nondeterministic collations.

Discussion: https://postgr.es/m/3605568.1731970579@sss.pgh.pa.us

src/backend/optimizer/prep/prepunion.c

@@ -570,9 +570,9 @@ generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
 
 	/*
 	 * If any of my children are identical UNION nodes (same op, all-flag, and
-	 * colTypes) then they can be merged into this node so that we generate
-	 * only one Append and unique-ification for the lot.  Recurse to find such
-	 * nodes and compute their children's paths.
+	 * colTypes/colCollations) then they can be merged into this node so that
+	 * we generate only one Append and unique-ification for the lot.  Recurse
+	 * to find such nodes and compute their children's paths.
 	 */
 	rellist = plan_union_children(root, op, refnames_tlist, &tlist_list);
 
@@ -861,17 +861,15 @@ generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
 }
 
 /*
- * Pull up children of a UNION node that are identically-propertied UNIONs.
+ * Pull up children of a UNION node that are identically-propertied UNIONs,
+ * and perform planning of the queries underneath the N-way UNION.
+ *
+ * The result is a list of RelOptInfos containing Paths for sub-nodes, with
+ * one entry for each descendant that is a leaf query or non-identical setop.
+ * We also return a parallel list of the childrens' targetlists.
  *
  * NOTE: we can also pull a UNION ALL up into a UNION, since the distinct
  * output rows will be lost anyway.
- *
- * NOTE: currently, we ignore collations while determining if a child has
- * the same properties.  This is semantically sound only so long as all
- * collations have the same notion of equality.  It is valid from an
- * implementation standpoint because we don't care about the ordering of
- * a UNION child's result: UNION ALL results are always unordered, and
- * generate_union_paths will force a fresh sort if the top level is a UNION.
  */
 static List *
 plan_union_children(PlannerInfo *root,
@@ -897,7 +895,8 @@ plan_union_children(PlannerInfo *root,
 
 			if (op->op == top_union->op &&
 				(op->all == top_union->all || op->all) &&
-				equal(op->colTypes, top_union->colTypes))
+				equal(op->colTypes, top_union->colTypes) &&
+				equal(op->colCollations, top_union->colCollations))
 			{
 				/* Same UNION, so fold children into parent */
 				pending_rels = lcons(op->rarg, pending_rels);