Fix usage of aggregate pathkeys in group_keys_reorder_by_pathkeys()

group_keys_reorder_by_pathkeys() function searched for matching pathkeys
within root->group_pathkeys.  That could lead to picking an aggregate pathkey
and using its pathkey->pk_eclass->ec_sortref as an argument of
get_sortgroupref_clause_noerr().  Given that ec_sortref of an aggregate pathkey
references aggregate targetlist not query targetlist, this leads to incorrect
query optimization.

Fix this by looking for matching pathkeys only within the first
num_groupby_pathkeys pathkeys.

Reported-by: David G. Johnston
Discussion: https://postgr.es/m/CAKFQuwY3Ek%3DcLThgd8FdaSc5JRDVt0FaV00gMcWra%2BTAR4gGUw%40mail.gmail.com
Author: Andrei Lepikhov, Alexander Korotkov

src/backend/optimizer/path/pathkeys.c

@@ -355,10 +355,14 @@ pathkeys_contained_in(List *keys1, List *keys2)
 
 /*
  * group_keys_reorder_by_pathkeys
- *		Reorder GROUP BY keys to match the input pathkeys.
+ *		Reorder GROUP BY pathkeys and clauses to match the input pathkeys.
  *
- * Function returns new lists (pathkeys and clauses), original GROUP BY lists
+ * 'pathkeys' is an input list of pathkeys
+ * '*group_pathkeys' and '*group_clauses' are pathkeys and clauses lists to
+ *		reorder.  The pointers are redirected to new lists, original lists
  *		stay untouched.
+ * 'num_groupby_pathkeys' is the number of first '*group_pathkeys' items to
+ *		search matching pathkeys.
  *
  * Returns the number of GROUP BY keys with a matching pathkey.
  */
@@ -369,12 +373,24 @@ group_keys_reorder_by_pathkeys(List *pathkeys, List **group_pathkeys,
 {
 	List	   *new_group_pathkeys = NIL,
 			   *new_group_clauses = NIL;
+	List	   *grouping_pathkeys;
 	ListCell   *lc;
 	int			n;
 
 	if (pathkeys == NIL || *group_pathkeys == NIL)
 		return 0;
 
+	/*
+	 * We're going to search within just the first num_groupby_pathkeys of
+	 * *group_pathkeys.  The thing is that root->group_pathkeys is passed as
+	 * *group_pathkeys containing grouping pathkeys altogether with aggregate
+	 * pathkeys.  If we process aggregate pathkeys we could get an invalid
+	 * result of get_sortgroupref_clause_noerr(), because their
+	 * pathkey->pk_eclass->ec_sortref doesn't referece query targetlist.  So,
+	 * we allocate a separate list of pathkeys for lookups.
+	 */
+	grouping_pathkeys = list_copy_head(*group_pathkeys, num_groupby_pathkeys);
+
 	/*
 	 * Walk the pathkeys (determining ordering of the input path) and see if
 	 * there's a matching GROUP BY key. If we find one, we append it to the
@@ -395,7 +411,7 @@ group_keys_reorder_by_pathkeys(List *pathkeys, List **group_pathkeys,
 		 * there is no sortclause reference for some reason.
 		 */
 		if (foreach_current_index(lc) >= num_groupby_pathkeys ||
-			!list_member_ptr(*group_pathkeys, pathkey) ||
+			!list_member_ptr(grouping_pathkeys, pathkey) ||
 			pathkey->pk_eclass->ec_sortref == 0)
 			break;
 
@@ -429,6 +445,7 @@ group_keys_reorder_by_pathkeys(List *pathkeys, List **group_pathkeys,
 	*group_clauses = list_concat_unique_ptr(new_group_clauses,
 											*group_clauses);
 
+	list_free(grouping_pathkeys);
 	return n;
 }
 

src/test/regress/expected/aggregates.out

@@ -2874,6 +2874,30 @@ SELECT y,x,array_agg(distinct w) FROM btg WHERE y < 0 GROUP BY x,y;
 
 RESET enable_incremental_sort;
 DROP TABLE btg;
+-- Check we don't pick aggregate path key instead of grouping path key
+CREATE TABLE group_agg_pk AS SELECT
+  i % 10 AS x,
+  i % 2 AS y,
+  i % 2 AS z,
+  2 AS w,
+  i % 10 AS f
+FROM generate_series(1,100) AS i;
+ANALYZE group_agg_pk;
+SET enable_nestloop = off;
+SET enable_hashjoin = off;
+SELECT
+  c1.z, c1.w, string_agg(''::text, repeat(''::text, c1.f) ORDER BY c1.x,c1.y)
+FROM group_agg_pk c1 JOIN group_agg_pk c2 ON (c1.x = c2.f)
+GROUP BY c1.w, c1.z;
+ z | w | string_agg 
+---+---+------------
+ 0 | 2 | 
+ 1 | 2 | 
+(2 rows)
+
+RESET enable_nestloop;
+RESET enable_hashjoin;
+DROP TABLE group_agg_pk;
 -- The case, when scanning sort order correspond to aggregate sort order but
 -- can not be found in the group-by list
 CREATE TABLE agg_sort_order (c1 int PRIMARY KEY, c2 int);

src/test/regress/sql/aggregates.sql

@@ -1231,6 +1231,25 @@ RESET enable_incremental_sort;
 
 DROP TABLE btg;
 
+-- Check we don't pick aggregate path key instead of grouping path key
+CREATE TABLE group_agg_pk AS SELECT
+  i % 10 AS x,
+  i % 2 AS y,
+  i % 2 AS z,
+  2 AS w,
+  i % 10 AS f
+FROM generate_series(1,100) AS i;
+ANALYZE group_agg_pk;
+SET enable_nestloop = off;
+SET enable_hashjoin = off;
+SELECT
+  c1.z, c1.w, string_agg(''::text, repeat(''::text, c1.f) ORDER BY c1.x,c1.y)
+FROM group_agg_pk c1 JOIN group_agg_pk c2 ON (c1.x = c2.f)
+GROUP BY c1.w, c1.z;
+RESET enable_nestloop;
+RESET enable_hashjoin;
+DROP TABLE group_agg_pk;
+
 -- The case, when scanning sort order correspond to aggregate sort order but
 -- can not be found in the group-by list
 CREATE TABLE agg_sort_order (c1 int PRIMARY KEY, c2 int);