3 files changed, 46 insertions, 3 deletions

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 1fbb0b28c3b..fb4fb987e7f 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3368,11 +3368,28 @@ double
 estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 					List **pgset, EstimationInfo *estinfo)
 {
-	List	   *varinfos = NIL;
+	return estimate_num_groups_incremental(root, groupExprs,
+										   input_rows, pgset, estinfo,
+										   NULL, 0);
+}
+
+/*
+ * estimate_num_groups_incremental
+ *		An estimate_num_groups variant, optimized for cases that are adding the
+ *		expressions incrementally (e.g. one by one).
+ */
+double
+estimate_num_groups_incremental(PlannerInfo *root, List *groupExprs,
+					double input_rows,
+					List **pgset, EstimationInfo *estinfo,
+					List **cache_varinfos, int prevNExprs)
+{
+	List	   *varinfos = (cache_varinfos) ? *cache_varinfos : NIL;
 	double		srf_multiplier = 1.0;
 	double		numdistinct;
 	ListCell   *l;
-	int			i;
+	int			i,
+				j;
 
 	/* Zero the estinfo output parameter, if non-NULL */
 	if (estinfo != NULL)
@@ -3403,7 +3420,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 	 */
 	numdistinct = 1.0;
 
-	i = 0;
+	i = j = 0;
 	foreach(l, groupExprs)
 	{
 		Node	   *groupexpr = (Node *) lfirst(l);
@@ -3412,6 +3429,14 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		List	   *varshere;
 		ListCell   *l2;
 
+		/* was done on previous call */
+		if (cache_varinfos && j++ < prevNExprs)
+		{
+			if (pgset)
+				i++; /* to keep in sync with lines below */
+			continue;
+		}
+
 		/* is expression in this grouping set? */
 		if (pgset && !list_member_int(*pgset, i++))
 			continue;
@@ -3481,7 +3506,11 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		if (varshere == NIL)
 		{
 			if (contain_volatile_functions(groupexpr))
+			{
+				if (cache_varinfos)
+					*cache_varinfos = varinfos;
 				return input_rows;
+			}
 			continue;
 		}
 
@@ -3498,6 +3527,9 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		}
 	}
 
+	if (cache_varinfos)
+		*cache_varinfos = varinfos;
+
 	/*
 	 * If now no Vars, we must have an all-constant or all-boolean GROUP BY
 	 * list.
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index eb3a03b9762..9e8ab1420d9 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1183,6 +1183,16 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 	{
+		{"enable_group_by_reordering", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("enable reordering of GROUP BY key"),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&enable_group_by_reordering,
+		true,
+		NULL, NULL, NULL
+	},
+	{
 		{"geqo", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Enables genetic query optimization."),
 			gettext_noop("This algorithm attempts to do planning without "
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index b933fade8c6..93d221a37b1 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -382,6 +382,7 @@
 #enable_seqscan = on
 #enable_sort = on
 #enable_tidscan = on
+#enable_group_by_reordering = on
 
 # - Planner Cost Constants -