Explore alternative orderings of group-by pathkeys during optimization.

When evaluating a query with a multi-column GROUP BY clause, we can minimize
sort operations or avoid them if we synchronize the order of GROUP BY clauses
with the ORDER BY sort clause or sort order, which comes from the underlying
query tree. Grouping does not imply any ordering, so we can compare
the keys in arbitrary order, and a Hash Agg leverages this. But for Group Agg,
we simply compared keys in the order specified in the query. This commit
explores alternative ordering of the keys, trying to find a cheaper one.

The ordering of group keys may interact with other parts of the query, some of
which may not be known while planning the grouping. For example, there may be
an explicit ORDER BY clause or some other ordering-dependent operation higher up
in the query, and using the same ordering may allow using either incremental
sort or even eliminating the sort entirely.

The patch always keeps the ordering specified in the query, assuming the user
might have additional insights.

This introduces a new GUC enable_group_by_reordering so that the optimization
may be disabled if needed.

Discussion: https://postgr.es/m/7c79e6a5-8597-74e8-0671-1c39d124c9d6%40sigaev.ru
Author: Andrei Lepikhov, Teodor Sigaev
Reviewed-by: Tomas Vondra, Claudio Freire, Gavin Flower, Dmitry Dolgov
Reviewed-by: Robert Haas, Pavel Borisov, David Rowley, Zhihong Yu
Reviewed-by: Tom Lane, Alexander Korotkov, Richard Guo, Alena Rybakina

src/backend/optimizer/path/equivclass.c

@@ -652,7 +652,18 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 
 			if (opcintype == cur_em->em_datatype &&
 				equal(expr, cur_em->em_expr))
-				return cur_ec;	/* Match! */
+			{
+				/*
+				 * 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;
+			}
 		}
 	}
 

src/backend/optimizer/path/pathkeys.c

@@ -22,12 +22,15 @@
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "nodes/plannodes.h"
+#include "optimizer/cost.h"
 #include "optimizer/optimizer.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "partitioning/partbounds.h"
 #include "utils/lsyscache.h"
 
+/* Consider reordering of GROUP BY keys? */
+bool		enable_group_by_reordering = true;
 
 static bool pathkey_is_redundant(PathKey *new_pathkey, List *pathkeys);
 static bool matches_boolean_partition_clause(RestrictInfo *rinfo,
@@ -350,6 +353,202 @@ pathkeys_contained_in(List *keys1, List *keys2)
 	return false;
 }
 
+/*
+ * group_keys_reorder_by_pathkeys
+ *		Reorder GROUP BY keys to match the input pathkeys.
+ *
+ * Function returns new lists (pathkeys and clauses), original GROUP BY lists
+ * stay untouched.
+ *
+ * Returns the number of GROUP BY keys with a matching pathkey.
+ */
+static int
+group_keys_reorder_by_pathkeys(List *pathkeys, List **group_pathkeys,
+							   List **group_clauses,
+							   int num_groupby_pathkeys)
+{
+	List	   *new_group_pathkeys = NIL,
+			   *new_group_clauses = NIL;
+	ListCell   *lc;
+	int			n;
+
+	if (pathkeys == NIL || *group_pathkeys == NIL)
+		return 0;
+
+	/*
+	 * 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
+	 * list, and do the same for the clauses.
+	 *
+	 * Once we find the first pathkey without a matching GROUP BY key, the
+	 * rest of the pathkeys are useless and can't be used to evaluate the
+	 * grouping, so we abort the loop and ignore the remaining pathkeys.
+	 */
+	foreach(lc, pathkeys)
+	{
+		PathKey    *pathkey = (PathKey *) lfirst(lc);
+		SortGroupClause *sgc;
+
+		/*
+		 * Pathkeys are built in a way that allows simply comparing pointers.
+		 * Give up if we can't find the matching pointer.  Also give up if
+		 * there is no sortclause reference for some reason.
+		 */
+		if (foreach_current_index(lc) >= num_groupby_pathkeys ||
+			!list_member_ptr(*group_pathkeys, pathkey) ||
+			pathkey->pk_eclass->ec_sortref == 0)
+			break;
+
+		/*
+		 * Since 1349d27 pathkey coming from underlying node can be in the
+		 * root->group_pathkeys but not in the processed_groupClause. So, we
+		 * should be careful here.
+		 */
+		sgc = get_sortgroupref_clause_noerr(pathkey->pk_eclass->ec_sortref,
+											*group_clauses);
+		if (!sgc)
+			/* The grouping clause does not cover this pathkey */
+			break;
+
+		/*
+		 * Sort group clause should have an ordering operator as long as there
+		 * is an associated pathkey.
+		 */
+		Assert(OidIsValid(sgc->sortop));
+
+		new_group_pathkeys = lappend(new_group_pathkeys, pathkey);
+		new_group_clauses = lappend(new_group_clauses, sgc);
+	}
+
+	/* remember the number of pathkeys with a matching GROUP BY key */
+	n = list_length(new_group_pathkeys);
+
+	/* append the remaining group pathkeys (will be treated as not sorted) */
+	*group_pathkeys = list_concat_unique_ptr(new_group_pathkeys,
+											 *group_pathkeys);
+	*group_clauses = list_concat_unique_ptr(new_group_clauses,
+											*group_clauses);
+
+	return n;
+}
+
+/*
+ * pathkeys_are_duplicate
+ *		Check if give pathkeys are already contained the list of
+ *		PathKeyInfo's.
+ */
+static bool
+pathkeys_are_duplicate(List *infos, List *pathkeys)
+{
+	ListCell   *lc;
+
+	foreach(lc, infos)
+	{
+		PathKeyInfo *info = lfirst_node(PathKeyInfo, lc);
+
+		if (compare_pathkeys(pathkeys, info->pathkeys) == PATHKEYS_EQUAL)
+			return true;
+	}
+	return false;
+}
+
+/*
+ * get_useful_group_keys_orderings
+ *		Determine which orderings of GROUP BY keys are potentially interesting.
+ *
+ * Returns a list of PathKeyInfo items, each representing an interesting
+ * ordering of GROUP BY keys.  Each item stores pathkeys and clauses in the
+ * matching order.
+ *
+ * The function considers (and keeps) multiple GROUP BY orderings:
+ *
+ * - the original ordering, as specified by the GROUP BY clause,
+ * - GROUP BY keys reordered to match 'path' ordering (as much as possible),
+ * - GROUP BY keys to match target ORDER BY clause (as much as possible).
+ */
+List *
+get_useful_group_keys_orderings(PlannerInfo *root, Path *path)
+{
+	Query	   *parse = root->parse;
+	List	   *infos = NIL;
+	PathKeyInfo *info;
+
+	List	   *pathkeys = root->group_pathkeys;
+	List	   *clauses = root->processed_groupClause;
+
+	/* always return at least the original pathkeys/clauses */
+	info = makeNode(PathKeyInfo);
+	info->pathkeys = pathkeys;
+	info->clauses = clauses;
+	infos = lappend(infos, info);
+
+	/*
+	 * Should we try generating alternative orderings of the group keys? If
+	 * not, we produce only the order specified in the query, i.e. the
+	 * optimization is effectively disabled.
+	 */
+	if (!enable_group_by_reordering)
+		return infos;
+
+	/*
+	 * Grouping sets have own and more complex logic to decide the ordering.
+	 */
+	if (parse->groupingSets)
+		return infos;
+
+	/*
+	 * If the path is sorted in some way, try reordering the group keys to
+	 * match the path as much of the ordering as possible.  Then thanks to
+	 * incremental sort we would get this sort as cheap as possible.
+	 */
+	if (path->pathkeys &&
+		!pathkeys_contained_in(path->pathkeys, root->group_pathkeys))
+	{
+		int			n;
+
+		n = group_keys_reorder_by_pathkeys(path->pathkeys, &pathkeys, &clauses,
+										   root->num_groupby_pathkeys);
+
+		if (n > 0 &&
+			(enable_incremental_sort || n == root->num_groupby_pathkeys) &&
+			!pathkeys_are_duplicate(infos, pathkeys))
+		{
+			info = makeNode(PathKeyInfo);
+			info->pathkeys = pathkeys;
+			info->clauses = clauses;
+
+			infos = lappend(infos, info);
+		}
+	}
+
+	/*
+	 * Try reordering pathkeys to minimize the sort cost (this time consider
+	 * the ORDER BY clause).
+	 */
+	if (root->sort_pathkeys &&
+		!pathkeys_contained_in(root->sort_pathkeys, root->group_pathkeys))
+	{
+		int			n;
+
+		n = group_keys_reorder_by_pathkeys(root->sort_pathkeys, &pathkeys,
+										   &clauses,
+										   root->num_groupby_pathkeys);
+
+		if (n > 0 &&
+			(enable_incremental_sort || n == list_length(root->sort_pathkeys)) &&
+			!pathkeys_are_duplicate(infos, pathkeys))
+		{
+			info = makeNode(PathKeyInfo);
+			info->pathkeys = pathkeys;
+			info->clauses = clauses;
+
+			infos = lappend(infos, info);
+		}
+	}
+
+	return infos;
+}
+
 /*
  * pathkeys_count_contained_in
  *    Same as pathkeys_contained_in, but also sets length of longest
@@ -1939,6 +2138,54 @@ pathkeys_useful_for_ordering(PlannerInfo *root, List *pathkeys)
 	return n_common_pathkeys;
 }
 
+/*
+ * pathkeys_useful_for_grouping
+ *		Count the number of pathkeys that are useful for grouping (instead of
+ *		explicit sort)
+ *
+ * Group pathkeys could be reordered to benefit from the ordering. The
+ * ordering may not be "complete" and may require incremental sort, but that's
+ * fine. So we simply count prefix pathkeys with a matching group key, and
+ * stop once we find the first pathkey without a match.
+ *
+ * So e.g. with pathkeys (a,b,c) and group keys (a,b,e) this determines (a,b)
+ * pathkeys are useful for grouping, and we might do incremental sort to get
+ * path ordered by (a,b,e).
+ *
+ * This logic is necessary to retain paths with ordering not matching grouping
+ * keys directly, without the reordering.
+ *
+ * Returns the length of pathkey prefix with matching group keys.
+ */
+static int
+pathkeys_useful_for_grouping(PlannerInfo *root, List *pathkeys)
+{
+	ListCell   *key;
+	int			n = 0;
+
+	/* no special ordering requested for grouping */
+	if (root->group_pathkeys == NIL)
+		return 0;
+
+	/* unordered path */
+	if (pathkeys == NIL)
+		return 0;
+
+	/* walk the pathkeys and search for matching group key */
+	foreach(key, pathkeys)
+	{
+		PathKey    *pathkey = (PathKey *) lfirst(key);
+
+		/* no matching group key, we're done */
+		if (!list_member_ptr(root->group_pathkeys, pathkey))
+			break;
+
+		n++;
+	}
+
+	return n;
+}
+
 /*
  * truncate_useless_pathkeys
  *		Shorten the given pathkey list to just the useful pathkeys.
@@ -1953,6 +2200,9 @@ truncate_useless_pathkeys(PlannerInfo *root,
 
 	nuseful = pathkeys_useful_for_merging(root, rel, pathkeys);
 	nuseful2 = pathkeys_useful_for_ordering(root, pathkeys);
+	if (nuseful2 > nuseful)
+		nuseful = nuseful2;
+	nuseful2 = pathkeys_useful_for_grouping(root, pathkeys);
 	if (nuseful2 > nuseful)
 		nuseful = nuseful2;
 
@@ -1988,6 +2238,8 @@ has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel)
 {
 	if (rel->joininfo != NIL || rel->has_eclass_joins)
 		return true;			/* might be able to use pathkeys for merging */
+	if (root->group_pathkeys != NIL)
+		return true;			/* might be able to use pathkeys for grouping */
 	if (root->query_pathkeys != NIL)
 		return true;			/* might be able to use them for ordering */
 	return false;				/* definitely useless */

src/backend/optimizer/plan/planner.c

@@ -140,7 +140,7 @@ static double preprocess_limit(PlannerInfo *root,
 							   double tuple_fraction,
 							   int64 *offset_est, int64 *count_est);
 static void remove_useless_groupby_columns(PlannerInfo *root);
-static List *preprocess_groupclause(PlannerInfo *root, List *force);
+static List *groupclause_apply_groupingset(PlannerInfo *root, List *force);
 static List *extract_rollup_sets(List *groupingSets);
 static List *reorder_grouping_sets(List *groupingSets, List *sortclause);
 static void standard_qp_callback(PlannerInfo *root, void *extra);
@@ -1423,7 +1423,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		else if (parse->groupClause)
 		{
 			/* Preprocess regular GROUP BY clause, if any */
-			root->processed_groupClause = preprocess_groupclause(root, NIL);
+			root->processed_groupClause = list_copy(parse->groupClause);;
 			/* Remove any redundant GROUP BY columns */
 			remove_useless_groupby_columns(root);
 		}
@@ -2144,7 +2144,7 @@ preprocess_grouping_sets(PlannerInfo *root)
 		 * The groupClauses for hashed grouping sets are built later on.)
 		 */
 		if (gs->set)
-			rollup->groupClause = preprocess_groupclause(root, gs->set);
+			rollup->groupClause = groupclause_apply_groupingset(root, gs->set);
 		else
 			rollup->groupClause = NIL;
 
@@ -2796,111 +2796,24 @@ remove_useless_groupby_columns(PlannerInfo *root)
 }
 
 /*
- * preprocess_groupclause - do preparatory work on GROUP BY clause
- *
- * The idea here is to adjust the ordering of the GROUP BY elements
- * (which in itself is semantically insignificant) to match ORDER BY,
- * thereby allowing a single sort operation to both implement the ORDER BY
- * requirement and set up for a Unique step that implements GROUP BY.
- *
- * In principle it might be interesting to consider other orderings of the
- * GROUP BY elements, which could match the sort ordering of other
- * possible plans (eg an indexscan) and thereby reduce cost.  We don't
- * bother with that, though.  Hashed grouping will frequently win anyway.
- *
- * Note: we need no comparable processing of the distinctClause because
- * the parser already enforced that that matches ORDER BY.
- *
- * Note: we return a fresh List, but its elements are the same
- * SortGroupClauses appearing in parse->groupClause.  This is important
- * because later processing may modify the processed_groupClause list.
- *
- * For grouping sets, the order of items is instead forced to agree with that
- * of the grouping set (and items not in the grouping set are skipped). The
- * work of sorting the order of grouping set elements to match the ORDER BY if
- * possible is done elsewhere.
+ * groupclause_apply_groupingset
+ *		Apply the order of GROUP BY clauses defined by grouping sets.  Items
+ *		not in the grouping set are skipped.
  */
 static List *
-preprocess_groupclause(PlannerInfo *root, List *force)
+groupclause_apply_groupingset(PlannerInfo *root, List *gset)
 {
 	Query	   *parse = root->parse;
 	List	   *new_groupclause = NIL;
-	bool		partial_match;
 	ListCell   *sl;
-	ListCell   *gl;
 
-	/* For grouping sets, we need to force the ordering */
-	if (force)
-	{
-		foreach(sl, force)
+	foreach(sl, gset)
 	{
 		Index		ref = lfirst_int(sl);
 		SortGroupClause *cl = get_sortgroupref_clause(ref, parse->groupClause);
 
 		new_groupclause = lappend(new_groupclause, cl);
 	}
-
-		return new_groupclause;
-	}
-
-	/* If no ORDER BY, nothing useful to do here */
-	if (parse->sortClause == NIL)
-		return list_copy(parse->groupClause);
-
-	/*
-	 * Scan the ORDER BY clause and construct a list of matching GROUP BY
-	 * items, but only as far as we can make a matching prefix.
-	 *
-	 * This code assumes that the sortClause contains no duplicate items.
-	 */
-	foreach(sl, parse->sortClause)
-	{
-		SortGroupClause *sc = lfirst_node(SortGroupClause, sl);
-
-		foreach(gl, parse->groupClause)
-		{
-			SortGroupClause *gc = lfirst_node(SortGroupClause, gl);
-
-			if (equal(gc, sc))
-			{
-				new_groupclause = lappend(new_groupclause, gc);
-				break;
-			}
-		}
-		if (gl == NULL)
-			break;				/* no match, so stop scanning */
-	}
-
-	/* Did we match all of the ORDER BY list, or just some of it? */
-	partial_match = (sl != NULL);
-
-	/* If no match at all, no point in reordering GROUP BY */
-	if (new_groupclause == NIL)
-		return list_copy(parse->groupClause);
-
-	/*
-	 * Add any remaining GROUP BY items to the new list, but only if we were
-	 * able to make a complete match.  In other words, we only rearrange the
-	 * GROUP BY list if the result is that one list is a prefix of the other
-	 * --- otherwise there's no possibility of a common sort.  Also, give up
-	 * if there are any non-sortable GROUP BY items, since then there's no
-	 * hope anyway.
-	 */
-	foreach(gl, parse->groupClause)
-	{
-		SortGroupClause *gc = lfirst_node(SortGroupClause, gl);
-
-		if (list_member_ptr(new_groupclause, gc))
-			continue;			/* it matched an ORDER BY item */
-		if (partial_match)		/* give up, no common sort possible */
-			return list_copy(parse->groupClause);
-		if (!OidIsValid(gc->sortop))	/* give up, GROUP BY can't be sorted */
-			return list_copy(parse->groupClause);
-		new_groupclause = lappend(new_groupclause, gc);
-	}
-
-	/* Success --- install the rearranged GROUP BY list */
-	Assert(list_length(parse->groupClause) == list_length(new_groupclause));
 	return new_groupclause;
 }
 
@@ -4200,7 +4113,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 			{
 				rollup = makeNode(RollupData);
 
-				rollup->groupClause = preprocess_groupclause(root, gset);
+				rollup->groupClause = groupclause_apply_groupingset(root, gset);
 				rollup->gsets_data = list_make1(gs);
 				rollup->gsets = remap_to_groupclause_idx(rollup->groupClause,
 														 rollup->gsets_data,
@@ -4389,7 +4302,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 
 			Assert(gs->set != NIL);
 
-			rollup->groupClause = preprocess_groupclause(root, gs->set);
+			rollup->groupClause = groupclause_apply_groupingset(root, gs->set);
 			rollup->gsets_data = list_make1(gs);
 			rollup->gsets = remap_to_groupclause_idx(rollup->groupClause,
 													 rollup->gsets_data,
@@ -6891,14 +6804,28 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 		 */
 		foreach(lc, input_rel->pathlist)
 		{
+			ListCell   *lc2;
 			Path	   *path = (Path *) lfirst(lc);
+			Path	   *path_save = path;
+			List	   *pathkey_orderings = NIL;
+
+			/* generate alternative group orderings that might be useful */
+			pathkey_orderings = get_useful_group_keys_orderings(root, path);
+
+			Assert(list_length(pathkey_orderings) > 0);
+
+			foreach(lc2, pathkey_orderings)
+			{
+				PathKeyInfo *info = (PathKeyInfo *) lfirst(lc2);
+
+				/* restore the path (we replace it in the loop) */
+				path = path_save;
 
 				path = make_ordered_path(root,
 										 grouped_rel,
 										 path,
 										 cheapest_path,
-									 root->group_pathkeys);
-
+										 info->pathkeys);
 				if (path == NULL)
 					continue;
 
@@ -6912,8 +6839,8 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 				else if (parse->hasAggs)
 				{
 					/*
-				 * We have aggregation, possibly with plain GROUP BY. Make an
-				 * AggPath.
+					 * We have aggregation, possibly with plain GROUP BY. Make
+					 * an AggPath.
 					 */
 					add_path(grouped_rel, (Path *)
 							 create_agg_path(root,
@@ -6922,7 +6849,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 											 grouped_rel->reltarget,
 											 parse->groupClause ? AGG_SORTED : AGG_PLAIN,
 											 AGGSPLIT_SIMPLE,
-										 root->processed_groupClause,
+											 info->clauses,
 											 havingQual,
 											 agg_costs,
 											 dNumGroups));
@@ -6930,14 +6857,14 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 				else if (parse->groupClause)
 				{
 					/*
-				 * We have GROUP BY without aggregation or grouping sets. Make
-				 * a GroupPath.
+					 * We have GROUP BY without aggregation or grouping sets.
+					 * Make a GroupPath.
 					 */
 					add_path(grouped_rel, (Path *)
 							 create_group_path(root,
 											   grouped_rel,
 											   path,
-										   root->processed_groupClause,
+											   info->clauses,
 											   havingQual,
 											   dNumGroups));
 				}
@@ -6947,6 +6874,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 					Assert(false);
 				}
 			}
+		}
 
 		/*
 		 * Instead of operating directly on the input relation, we can
@@ -6956,13 +6884,29 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 		{
 			foreach(lc, partially_grouped_rel->pathlist)
 			{
+				ListCell   *lc2;
 				Path	   *path = (Path *) lfirst(lc);
+				Path	   *path_save = path;
+				List	   *pathkey_orderings = NIL;
+
+				/* generate alternative group orderings that might be useful */
+				pathkey_orderings = get_useful_group_keys_orderings(root, path);
+
+				Assert(list_length(pathkey_orderings) > 0);
+
+				/* process all potentially interesting grouping reorderings */
+				foreach(lc2, pathkey_orderings)
+				{
+					PathKeyInfo *info = (PathKeyInfo *) lfirst(lc2);
+
+					/* restore the path (we replace it in the loop) */
+					path = path_save;
 
 					path = make_ordered_path(root,
 											 grouped_rel,
 											 path,
 											 partially_grouped_rel->cheapest_total_path,
-										 root->group_pathkeys);
+											 info->pathkeys);
 
 					if (path == NULL)
 						continue;
@@ -6975,7 +6919,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 												 grouped_rel->reltarget,
 												 parse->groupClause ? AGG_SORTED : AGG_PLAIN,
 												 AGGSPLIT_FINAL_DESERIAL,
-											 root->processed_groupClause,
+												 info->clauses,
 												 havingQual,
 												 agg_final_costs,
 												 dNumGroups));
@@ -6984,13 +6928,14 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 								 create_group_path(root,
 												   grouped_rel,
 												   path,
-											   root->processed_groupClause,
+												   info->clauses,
 												   havingQual,
 												   dNumGroups));
 
 				}
 			}
 		}
+	}
 
 	if (can_hash)
 	{
@@ -7190,13 +7135,29 @@ create_partial_grouping_paths(PlannerInfo *root,
 		 */
 		foreach(lc, input_rel->pathlist)
 		{
+			ListCell   *lc2;
 			Path	   *path = (Path *) lfirst(lc);
+			Path	   *path_save = path;
+			List	   *pathkey_orderings = NIL;
+
+			/* generate alternative group orderings that might be useful */
+			pathkey_orderings = get_useful_group_keys_orderings(root, path);
+
+			Assert(list_length(pathkey_orderings) > 0);
+
+			/* process all potentially interesting grouping reorderings */
+			foreach(lc2, pathkey_orderings)
+			{
+				PathKeyInfo *info = (PathKeyInfo *) lfirst(lc2);
+
+				/* restore the path (we replace it in the loop) */
+				path = path_save;
 
 				path = make_ordered_path(root,
 										 partially_grouped_rel,
 										 path,
 										 cheapest_total_path,
-									 root->group_pathkeys);
+										 info->pathkeys);
 
 				if (path == NULL)
 					continue;
@@ -7209,7 +7170,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 											 partially_grouped_rel->reltarget,
 											 parse->groupClause ? AGG_SORTED : AGG_PLAIN,
 											 AGGSPLIT_INITIAL_SERIAL,
-										 root->processed_groupClause,
+											 info->clauses,
 											 NIL,
 											 agg_partial_costs,
 											 dNumPartialGroups));
@@ -7218,24 +7179,42 @@ create_partial_grouping_paths(PlannerInfo *root,
 							 create_group_path(root,
 											   partially_grouped_rel,
 											   path,
-										   root->processed_groupClause,
+											   info->clauses,
 											   NIL,
 											   dNumPartialGroups));
 			}
 		}
+	}
 
 	if (can_sort && cheapest_partial_path != NULL)
 	{
 		/* Similar to above logic, but for partial paths. */
 		foreach(lc, input_rel->partial_pathlist)
 		{
+			ListCell   *lc2;
 			Path	   *path = (Path *) lfirst(lc);
+			Path	   *path_save = path;
+			List	   *pathkey_orderings = NIL;
+
+			/* generate alternative group orderings that might be useful */
+			pathkey_orderings = get_useful_group_keys_orderings(root, path);
+
+			Assert(list_length(pathkey_orderings) > 0);
+
+			/* process all potentially interesting grouping reorderings */
+			foreach(lc2, pathkey_orderings)
+			{
+				PathKeyInfo *info = (PathKeyInfo *) lfirst(lc2);
+
+
+				/* restore the path (we replace it in the loop) */
+				path = path_save;
 
 				path = make_ordered_path(root,
 										 partially_grouped_rel,
 										 path,
 										 cheapest_partial_path,
-									 root->group_pathkeys);
+										 info->pathkeys);
 
 				if (path == NULL)
 					continue;
@@ -7248,7 +7227,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 													 partially_grouped_rel->reltarget,
 													 parse->groupClause ? AGG_SORTED : AGG_PLAIN,
 													 AGGSPLIT_INITIAL_SERIAL,
-												 root->processed_groupClause,
+													 info->clauses,
 													 NIL,
 													 agg_partial_costs,
 													 dNumPartialPartialGroups));
@@ -7257,11 +7236,12 @@ create_partial_grouping_paths(PlannerInfo *root,
 									 create_group_path(root,
 													   partially_grouped_rel,
 													   path,
-												   root->processed_groupClause,
+													   info->clauses,
 													   NIL,
 													   dNumPartialPartialGroups));
 			}
 		}
+	}
 
 	/*
 	 * Add a partially-grouped HashAgg Path where possible
@@ -7373,6 +7353,8 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 	 * We can also skip the entire loop when we only have a single-item
 	 * group_pathkeys because then we can't possibly have a presorted prefix
 	 * of the list without having the list be fully sorted.
+	 *
+	 * XXX Shouldn't this also consider the group-key-reordering?
 	 */
 	if (!enable_incremental_sort || list_length(root->group_pathkeys) == 1)
 		return;

src/backend/utils/misc/guc_tables.c

@@ -1050,6 +1050,16 @@ struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+	{
+		{"enable_group_by_reordering", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enables reordering of GROUP BY keys."),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&enable_group_by_reordering,
+		true,
+		NULL, NULL, NULL
+	},
 	{
 		{"geqo", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Enables genetic query optimization."),

src/backend/utils/misc/postgresql.conf.sample

@@ -399,6 +399,7 @@
 #enable_seqscan = on
 #enable_sort = on
 #enable_tidscan = on
+#enable_group_by_reordering = on
 
 # - Planner Cost Constants -
 

src/include/nodes/pathnodes.h

@@ -1456,6 +1456,16 @@ typedef struct PathKey
 	bool		pk_nulls_first; /* do NULLs come before normal values? */
 } PathKey;
 
+/*
+ * Combines the information about pathkeys and the associated clauses.
+ */
+typedef struct PathKeyInfo
+{
+	NodeTag		type;
+	List	   *pathkeys;
+	List	   *clauses;
+} PathKeyInfo;
+
 /*
  * VolatileFunctionStatus -- allows nodes to cache their
  * contain_volatile_functions properties. VOLATILITY_UNKNOWN means not yet

src/include/optimizer/paths.h

@@ -24,6 +24,7 @@ extern PGDLLIMPORT bool enable_geqo;
 extern PGDLLIMPORT int geqo_threshold;
 extern PGDLLIMPORT int min_parallel_table_scan_size;
 extern PGDLLIMPORT int min_parallel_index_scan_size;
+extern PGDLLIMPORT bool enable_group_by_reordering;
 
 /* Hook for plugins to get control in set_rel_pathlist() */
 typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
@@ -204,6 +205,7 @@ typedef enum
 extern PathKeysComparison compare_pathkeys(List *keys1, List *keys2);
 extern bool pathkeys_contained_in(List *keys1, List *keys2);
 extern bool pathkeys_count_contained_in(List *keys1, List *keys2, int *n_common);
+extern List *get_useful_group_keys_orderings(PlannerInfo *root, Path *path);
 extern Path *get_cheapest_path_for_pathkeys(List *paths, List *pathkeys,
 											Relids required_outer,
 											CostSelector cost_criterion,

src/test/regress/expected/aggregates.out

@@ -2728,6 +2728,208 @@ SELECT balk(hundred) FROM tenk1;
 (1 row)
 
 ROLLBACK;
+-- GROUP BY optimization by reorder columns
+CREATE TABLE btg AS SELECT
+  i % 100 AS x,
+  i % 100 AS y,
+  'abc' || i % 10 AS z,
+  i AS w
+FROM generate_series(1,10000) AS i;
+CREATE INDEX abc ON btg(x,y);
+ANALYZE btg;
+-- GROUP BY optimization by reorder columns by frequency
+SET enable_hashagg=off;
+SET max_parallel_workers= 0;
+SET max_parallel_workers_per_gather = 0;
+-- Utilize index scan ordering to avoid a Sort operation
+EXPLAIN (COSTS OFF) SELECT count(*) FROM btg GROUP BY x,y;
+               QUERY PLAN               
+----------------------------------------
+ GroupAggregate
+   Group Key: x, y
+   ->  Index Only Scan using abc on btg
+(3 rows)
+
+EXPLAIN (COSTS OFF) SELECT count(*) FROM btg GROUP BY y,x;
+               QUERY PLAN               
+----------------------------------------
+ GroupAggregate
+   Group Key: x, y
+   ->  Index Only Scan using abc on btg
+(3 rows)
+
+-- Engage incremental sort
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY x,y,z,w;
+               QUERY PLAN                
+-----------------------------------------
+ Group
+   Group Key: x, y, z, w
+   ->  Incremental Sort
+         Sort Key: x, y, z, w
+         Presorted Key: x, y
+         ->  Index Scan using abc on btg
+(6 rows)
+
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY z,y,w,x;
+               QUERY PLAN                
+-----------------------------------------
+ Group
+   Group Key: x, y, z, w
+   ->  Incremental Sort
+         Sort Key: x, y, z, w
+         Presorted Key: x, y
+         ->  Index Scan using abc on btg
+(6 rows)
+
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,z,x,y;
+               QUERY PLAN                
+-----------------------------------------
+ Group
+   Group Key: x, y, w, z
+   ->  Incremental Sort
+         Sort Key: x, y, w, z
+         Presorted Key: x, y
+         ->  Index Scan using abc on btg
+(6 rows)
+
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,x,z,y;
+               QUERY PLAN                
+-----------------------------------------
+ Group
+   Group Key: x, y, w, z
+   ->  Incremental Sort
+         Sort Key: x, y, w, z
+         Presorted Key: x, y
+         ->  Index Scan using abc on btg
+(6 rows)
+
+-- Subqueries
+explain (COSTS OFF) SELECT x,y
+FROM (SELECT * FROM btg ORDER BY x,y,w,z) AS q1
+GROUP BY (w,x,z,y);
+                  QUERY PLAN                  
+----------------------------------------------
+ Group
+   Group Key: btg.x, btg.y, btg.w, btg.z
+   ->  Incremental Sort
+         Sort Key: btg.x, btg.y, btg.w, btg.z
+         Presorted Key: btg.x, btg.y
+         ->  Index Scan using abc on btg
+(6 rows)
+
+explain (COSTS OFF) SELECT x,y
+FROM (SELECT * FROM btg ORDER BY x,y,w,z LIMIT 100) AS q1
+GROUP BY (w,x,z,y);
+                     QUERY PLAN                     
+----------------------------------------------------
+ Group
+   Group Key: btg.x, btg.y, btg.w, btg.z
+   ->  Limit
+         ->  Incremental Sort
+               Sort Key: btg.x, btg.y, btg.w, btg.z
+               Presorted Key: btg.x, btg.y
+               ->  Index Scan using abc on btg
+(7 rows)
+
+-- Should work with and without GROUP-BY optimization
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,x,z,y ORDER BY y,x,z,w;
+          QUERY PLAN          
+------------------------------
+ Group
+   Group Key: y, x, z, w
+   ->  Sort
+         Sort Key: y, x, z, w
+         ->  Seq Scan on btg
+(5 rows)
+
+-- Utilize incremental sort to make the ORDER BY rule a bit cheaper
+explain (COSTS OFF) SELECT x,w FROM btg GROUP BY w,x,y,z ORDER BY x*x,z;
+                  QUERY PLAN                   
+-----------------------------------------------
+ Sort
+   Sort Key: ((x * x)), z
+   ->  Group
+         Group Key: x, y, w, z
+         ->  Incremental Sort
+               Sort Key: x, y, w, z
+               Presorted Key: x, y
+               ->  Index Scan using abc on btg
+(8 rows)
+
+SET enable_incremental_sort = off;
+-- The case when the number of incoming subtree path keys is more than
+-- the number of grouping keys.
+CREATE INDEX idx_y_x_z ON btg(y,x,w);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT y,x,array_agg(distinct w) FROM btg WHERE y < 0 GROUP BY x,y;
+                     QUERY PLAN                      
+-----------------------------------------------------
+ GroupAggregate
+   Output: y, x, array_agg(DISTINCT w)
+   Group Key: btg.y, btg.x
+   ->  Index Only Scan using idx_y_x_z on public.btg
+         Output: y, x, w
+         Index Cond: (btg.y < 0)
+(6 rows)
+
+RESET enable_incremental_sort;
+DROP TABLE btg;
+-- The case, when scanning sort order correspond to aggregate sort order but
+-- can not be found in the group-by list
+CREATE TABLE t1 (c1 int PRIMARY KEY, c2 int);
+CREATE UNIQUE INDEX ON t1(c2);
+explain (costs off)
+SELECT array_agg(c1 ORDER BY c2),c2
+FROM t1 WHERE c2 < 100 GROUP BY c1 ORDER BY 2;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Sort
+   Sort Key: c2
+   ->  GroupAggregate
+         Group Key: c1
+         ->  Sort
+               Sort Key: c1, c2
+               ->  Bitmap Heap Scan on t1
+                     Recheck Cond: (c2 < 100)
+                     ->  Bitmap Index Scan on t1_c2_idx
+                           Index Cond: (c2 < 100)
+(10 rows)
+
+DROP TABLE t1 CASCADE;
+-- Check, that GROUP-BY reordering optimization can operate with pathkeys, built
+-- by planner itself. For example, by MergeJoin.
+SET enable_hashjoin = off;
+SET enable_nestloop = off;
+explain (COSTS OFF)
+SELECT c1.relname,c1.relpages
+FROM pg_class c1 JOIN pg_class c2 ON (c1.relname=c2.relname AND c1.relpages=c2.relpages)
+GROUP BY c1.reltuples,c1.relpages,c1.relname
+ORDER BY c1.relpages, c1.relname, c1.relpages*c1.relpages;
+                                         QUERY PLAN                                          
+---------------------------------------------------------------------------------------------
+ Incremental Sort
+   Sort Key: c1.relpages, c1.relname, ((c1.relpages * c1.relpages))
+   Presorted Key: c1.relpages, c1.relname
+   ->  Group
+         Group Key: c1.relpages, c1.relname, c1.reltuples
+         ->  Incremental Sort
+               Sort Key: c1.relpages, c1.relname, c1.reltuples
+               Presorted Key: c1.relpages, c1.relname
+               ->  Merge Join
+                     Merge Cond: ((c1.relpages = c2.relpages) AND (c1.relname = c2.relname))
+                     ->  Sort
+                           Sort Key: c1.relpages, c1.relname
+                           ->  Seq Scan on pg_class c1
+                     ->  Sort
+                           Sort Key: c2.relpages, c2.relname
+                           ->  Seq Scan on pg_class c2
+(16 rows)
+
+RESET enable_hashjoin;
+RESET enable_nestloop;
+RESET enable_hashagg;
+RESET max_parallel_workers;
+RESET max_parallel_workers_per_gather;
 -- Secondly test the case of a parallel aggregate combiner function
 -- returning NULL. For that use normal transition function, but a
 -- combiner function returning NULL.

src/test/regress/expected/sysviews.out

@@ -114,6 +114,7 @@ select name, setting from pg_settings where name like 'enable%';
  enable_async_append            | on
  enable_bitmapscan              | on
  enable_gathermerge             | on
+ enable_group_by_reordering     | on
  enable_hashagg                 | on
  enable_hashjoin                | on
  enable_incremental_sort        | on
@@ -133,7 +134,7 @@ select name, setting from pg_settings where name like 'enable%';
  enable_seqscan                 | on
  enable_sort                    | on
  enable_tidscan                 | on
-(22 rows)
+(23 rows)
 
 -- There are always wait event descriptions for various types.
 select type, count(*) > 0 as ok FROM pg_wait_events

src/test/regress/sql/aggregates.sql

@@ -1181,6 +1181,81 @@ SELECT balk(hundred) FROM tenk1;
 
 ROLLBACK;
 
+-- GROUP BY optimization by reorder columns
+CREATE TABLE btg AS SELECT
+  i % 100 AS x,
+  i % 100 AS y,
+  'abc' || i % 10 AS z,
+  i AS w
+FROM generate_series(1,10000) AS i;
+CREATE INDEX abc ON btg(x,y);
+ANALYZE btg;
+
+-- GROUP BY optimization by reorder columns by frequency
+
+SET enable_hashagg=off;
+SET max_parallel_workers= 0;
+SET max_parallel_workers_per_gather = 0;
+
+-- Utilize index scan ordering to avoid a Sort operation
+EXPLAIN (COSTS OFF) SELECT count(*) FROM btg GROUP BY x,y;
+EXPLAIN (COSTS OFF) SELECT count(*) FROM btg GROUP BY y,x;
+
+-- Engage incremental sort
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY x,y,z,w;
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY z,y,w,x;
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,z,x,y;
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,x,z,y;
+
+-- Subqueries
+explain (COSTS OFF) SELECT x,y
+FROM (SELECT * FROM btg ORDER BY x,y,w,z) AS q1
+GROUP BY (w,x,z,y);
+explain (COSTS OFF) SELECT x,y
+FROM (SELECT * FROM btg ORDER BY x,y,w,z LIMIT 100) AS q1
+GROUP BY (w,x,z,y);
+
+-- Should work with and without GROUP-BY optimization
+explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,x,z,y ORDER BY y,x,z,w;
+
+-- Utilize incremental sort to make the ORDER BY rule a bit cheaper
+explain (COSTS OFF) SELECT x,w FROM btg GROUP BY w,x,y,z ORDER BY x*x,z;
+
+SET enable_incremental_sort = off;
+-- The case when the number of incoming subtree path keys is more than
+-- the number of grouping keys.
+CREATE INDEX idx_y_x_z ON btg(y,x,w);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT y,x,array_agg(distinct w) FROM btg WHERE y < 0 GROUP BY x,y;
+RESET enable_incremental_sort;
+
+DROP TABLE btg;
+
+-- The case, when scanning sort order correspond to aggregate sort order but
+-- can not be found in the group-by list
+CREATE TABLE t1 (c1 int PRIMARY KEY, c2 int);
+CREATE UNIQUE INDEX ON t1(c2);
+explain (costs off)
+SELECT array_agg(c1 ORDER BY c2),c2
+FROM t1 WHERE c2 < 100 GROUP BY c1 ORDER BY 2;
+DROP TABLE t1 CASCADE;
+
+-- Check, that GROUP-BY reordering optimization can operate with pathkeys, built
+-- by planner itself. For example, by MergeJoin.
+SET enable_hashjoin = off;
+SET enable_nestloop = off;
+explain (COSTS OFF)
+SELECT c1.relname,c1.relpages
+FROM pg_class c1 JOIN pg_class c2 ON (c1.relname=c2.relname AND c1.relpages=c2.relpages)
+GROUP BY c1.reltuples,c1.relpages,c1.relname
+ORDER BY c1.relpages, c1.relname, c1.relpages*c1.relpages;
+RESET enable_hashjoin;
+RESET enable_nestloop;
+
+RESET enable_hashagg;
+RESET max_parallel_workers;
+RESET max_parallel_workers_per_gather;
+
 -- Secondly test the case of a parallel aggregate combiner function
 -- returning NULL. For that use normal transition function, but a
 -- combiner function returning NULL.

src/tools/pgindent/typedefs.list

@@ -4045,3 +4045,4 @@ manifest_writer
 rfile
 ws_options
 ws_file_info
+PathKeyInfo