Fix asymmetry in setting EquivalenceClass.ec_sortref

0452b461bc made get_eclass_for_sort_expr() always set
EquivalenceClass.ec_sortref if it's not done yet.  This leads to an asymmetric
situation when whoever first looks for the EquivalenceClass sets the
ec_sortref.  It is also counterintuitive that get_eclass_for_sort_expr()
performs modification of data structures.

This commit makes make_pathkeys_for_sortclauses_extended() responsible for
setting EquivalenceClass.ec_sortref.  Now we set the
EquivalenceClass.ec_sortref's needed to explore alternative GROUP BY ordering
specifically during building pathkeys by the list of grouping clauses.

Discussion: https://postgr.es/m/17037754-f187-4138-8285-0e2bfebd0dea%40postgrespro.ru
Reported-by: Tom Lane
Author: Andrei Lepikhov
Reviewed-by: Alexander Korotkov, Pavel Borisov

src/backend/optimizer/path/equivclass.c

@@ -652,18 +652,7 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 
 			if (opcintype == cur_em->em_datatype &&
 				equal(expr, cur_em->em_expr))
-			{
-				/*
-				 * Match!
-				 *
-				 * Copy the sortref if it wasn't set yet.  That may happen if
-				 * the ec was constructed from a WHERE clause, i.e. it doesn't
-				 * have a target reference at all.
-				 */
-				if (cur_ec->ec_sortref == 0 && sortref > 0)
-					cur_ec->ec_sortref = sortref;
-				return cur_ec;
-			}
+				return cur_ec;	/* Match! */
 		}
 	}
 

src/backend/optimizer/path/pathkeys.c

@@ -1355,7 +1355,8 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
 													&sortclauses,
 													tlist,
 													false,
-													&sortable);
+													&sortable,
+													false);
 	/* It's caller error if not all clauses were sortable */
 	Assert(sortable);
 	return result;
@@ -1379,13 +1380,17 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
  * to remove any clauses that can be proven redundant via the eclass logic.
  * Even though we'll have to hash in that case, we might as well not hash
  * redundant columns.)
+ *
+ * If set_ec_sortref is true then sets the value of the pathkey's
+ * EquivalenceClass unless it's already initialized.
  */
 List *
 make_pathkeys_for_sortclauses_extended(PlannerInfo *root,
 									   List **sortclauses,
 									   List *tlist,
 									   bool remove_redundant,
-									   bool *sortable)
+									   bool *sortable,
+									   bool set_ec_sortref)
 {
 	List	   *pathkeys = NIL;
 	ListCell   *l;
@@ -1409,6 +1414,15 @@ make_pathkeys_for_sortclauses_extended(PlannerInfo *root,
 										   sortcl->nulls_first,
 										   sortcl->tleSortGroupRef,
 										   true);
+		if (pathkey->pk_eclass->ec_sortref == 0 && set_ec_sortref)
+		{
+			/*
+			 * Copy the sortref if it hasn't been set yet.  That may happen if
+			 * the EquivalenceClass was constructed from a WHERE clause, i.e.
+			 * it doesn't have a target reference at all.
+			 */
+			pathkey->pk_eclass->ec_sortref = sortcl->tleSortGroupRef;
+		}
 
 		/* Canonical form eliminates redundant ordering keys */
 		if (!pathkey_is_redundant(pathkey, pathkeys))

src/backend/optimizer/plan/planner.c

@@ -3395,12 +3395,17 @@ standard_qp_callback(PlannerInfo *root, void *extra)
 		 */
 		bool		sortable;
 
+		/*
+		 * Convert group clauses into pathkeys.  Set the ec_sortref field of
+		 * EquivalenceClass'es if it's not set yet.
+		 */
 		root->group_pathkeys =
 			make_pathkeys_for_sortclauses_extended(root,
 												   &root->processed_groupClause,
 												   tlist,
 												   true,
-												   &sortable);
+												   &sortable,
+												   true);
 		if (!sortable)
 		{
 			/* Can't sort; no point in considering aggregate ordering either */
@@ -3450,7 +3455,8 @@ standard_qp_callback(PlannerInfo *root, void *extra)
 												   &root->processed_distinctClause,
 												   tlist,
 												   true,
-												   &sortable);
+												   &sortable,
+												   false);
 		if (!sortable)
 			root->distinct_pathkeys = NIL;
 	}
@@ -3476,7 +3482,8 @@ standard_qp_callback(PlannerInfo *root, void *extra)
 												   &groupClauses,
 												   tlist,
 												   false,
-												   &sortable);
+												   &sortable,
+												   false);
 		if (!sortable)
 			root->setop_pathkeys = NIL;
 	}
@@ -6061,7 +6068,8 @@ make_pathkeys_for_window(PlannerInfo *root, WindowClause *wc,
 																 &wc->partitionClause,
 																 tlist,
 																 true,
-																 &sortable);
+																 &sortable,
+																 false);
 
 		Assert(sortable);
 	}

src/include/optimizer/paths.h

@@ -239,7 +239,8 @@ extern List *make_pathkeys_for_sortclauses_extended(PlannerInfo *root,
 													List **sortclauses,
 													List *tlist,
 													bool remove_redundant,
-													bool *sortable);
+													bool *sortable,
+													bool set_ec_sortref);
 extern void initialize_mergeclause_eclasses(PlannerInfo *root,
 											RestrictInfo *restrictinfo);
 extern void update_mergeclause_eclasses(PlannerInfo *root,

src/test/regress/expected/aggregates.out

@@ -2917,6 +2917,53 @@ GROUP BY c1.w, c1.z;
  5.0000000000000000
 (2 rows)
 
+-- Pathkeys, built in a subtree, can be used to optimize GROUP-BY clause
+-- ordering.  Also, here we check that it doesn't depend on the initial clause
+-- order in the GROUP-BY list.
+EXPLAIN (COSTS OFF)
+SELECT c1.y,c1.x FROM group_agg_pk c1
+  JOIN group_agg_pk c2
+  ON c1.x = c2.x
+GROUP BY c1.y,c1.x,c2.x;
+                     QUERY PLAN                      
+-----------------------------------------------------
+ Group
+   Group Key: c1.x, c1.y
+   ->  Incremental Sort
+         Sort Key: c1.x, c1.y
+         Presorted Key: c1.x
+         ->  Merge Join
+               Merge Cond: (c1.x = c2.x)
+               ->  Sort
+                     Sort Key: c1.x
+                     ->  Seq Scan on group_agg_pk c1
+               ->  Sort
+                     Sort Key: c2.x
+                     ->  Seq Scan on group_agg_pk c2
+(13 rows)
+
+EXPLAIN (COSTS OFF)
+SELECT c1.y,c1.x FROM group_agg_pk c1
+  JOIN group_agg_pk c2
+  ON c1.x = c2.x
+GROUP BY c1.y,c2.x,c1.x;
+                     QUERY PLAN                      
+-----------------------------------------------------
+ Group
+   Group Key: c2.x, c1.y
+   ->  Incremental Sort
+         Sort Key: c2.x, c1.y
+         Presorted Key: c2.x
+         ->  Merge Join
+               Merge Cond: (c1.x = c2.x)
+               ->  Sort
+                     Sort Key: c1.x
+                     ->  Seq Scan on group_agg_pk c1
+               ->  Sort
+                     Sort Key: c2.x
+                     ->  Seq Scan on group_agg_pk c2
+(13 rows)
+
 RESET enable_nestloop;
 RESET enable_hashjoin;
 DROP TABLE group_agg_pk;

src/test/regress/sql/aggregates.sql

@@ -1263,6 +1263,20 @@ SELECT avg(c1.f ORDER BY c1.x, c1.y)
 FROM group_agg_pk c1 JOIN group_agg_pk c2 ON c1.x = c2.x
 GROUP BY c1.w, c1.z;
 
+-- Pathkeys, built in a subtree, can be used to optimize GROUP-BY clause
+-- ordering.  Also, here we check that it doesn't depend on the initial clause
+-- order in the GROUP-BY list.
+EXPLAIN (COSTS OFF)
+SELECT c1.y,c1.x FROM group_agg_pk c1
+  JOIN group_agg_pk c2
+  ON c1.x = c2.x
+GROUP BY c1.y,c1.x,c2.x;
+EXPLAIN (COSTS OFF)
+SELECT c1.y,c1.x FROM group_agg_pk c1
+  JOIN group_agg_pk c2
+  ON c1.x = c2.x
+GROUP BY c1.y,c2.x,c1.x;
+
 RESET enable_nestloop;
 RESET enable_hashjoin;
 DROP TABLE group_agg_pk;