Respect enable_hashagg in the master planner for group by

src/backend/distributed/planner/multi_master_planner.c

@@ -21,6 +21,7 @@
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "optimizer/clauses.h"
+#include "optimizer/cost.h"
 #include "optimizer/planmain.h"
 #include "optimizer/tlist.h"
 #include "optimizer/var.h"
@@ -128,13 +129,47 @@ BuildAggregatePlan(Query *masterQuery, Plan *subPlan)
 	/* if we have grouping, then initialize appropriate information */
 	if (groupColumnCount > 0)
 	{
-		if (!grouping_is_hashable(groupColumnList))
+		bool groupingIsHashable = grouping_is_hashable(groupColumnList);
+		bool groupingIsSortable = grouping_is_sortable(groupColumnList);
+
+		if (!groupingIsHashable && !groupingIsSortable)
 		{
-			ereport(ERROR, (errmsg("grouped column list cannot be hashed")));
+			ereport(ERROR, (errmsg("grouped column list cannot be hashed or sorted")));
 		}
 
-		/* switch to hashed aggregate strategy to allow grouping */
+		/*
+		 * Postgres hash aggregate strategy does not support distinct aggregates
+		 * in group and order by with aggregate operations.
+		 * see nodeAgg.c:build_pertrans_for_aggref(). In that case we use
+		 * sorted agg strategy, otherwise we use hash strategy.
+		 */
+		if (!enable_hashagg || !groupingIsHashable)
+		{
+			char *messageHint = NULL;
+			if (!enable_hashagg && groupingIsHashable)
+			{
+				messageHint = "Consider setting enable_hashagg to on.";
+			}
+
+			if (!groupingIsSortable)
+			{
+				ereport(ERROR, (errmsg("grouped column list must cannot be sorted"),
+								errdetail("Having a distinct aggregate requires "
+										  "grouped column list to be sortable."),
+								messageHint ? errhint("%s", messageHint) : 0));
+			}
+
+			aggregateStrategy = AGG_SORTED;
+#if (PG_VERSION_NUM >= 90600)
+			subPlan = (Plan *) make_sort_from_sortclauses(groupColumnList, subPlan);
+#else
+			subPlan = (Plan *) make_sort_from_sortclauses(NULL, groupColumnList, subPlan);
+#endif
+		}
+		else
+		{
 			aggregateStrategy = AGG_HASHED;
+		}
 
 		/* get column indexes that are being grouped */
 		groupColumnIdArray = extract_grouping_cols(groupColumnList, subPlan->targetlist);

src/test/regress/expected/multi_explain.out

@@ -686,3 +686,23 @@ Custom Scan (Citus Router)
         ->  Seq Scan on explain_table_570001 explain_table
               Filter: (id = 1)
 ROLLBACK;
+-- Test disable hash aggregate
+SET enable_hashagg TO off;
+EXPLAIN (COSTS FALSE, FORMAT TEXT)
+	SELECT l_quantity, count(*) count_quantity FROM lineitem
+	GROUP BY l_quantity ORDER BY count_quantity, l_quantity;
+Sort
+  Sort Key: COALESCE((pg_catalog.sum((COALESCE((pg_catalog.sum(remote_scan.count_quantity))::bigint, '0'::bigint))))::bigint, '0'::bigint), remote_scan.l_quantity
+  ->  GroupAggregate
+        Group Key: remote_scan.l_quantity
+        ->  Sort
+              Sort Key: remote_scan.l_quantity
+              ->  Custom Scan (Citus Real-Time)
+                    Task Count: 8
+                    Tasks Shown: One of 8
+                    ->  Task
+                          Node: host=localhost port=57637 dbname=regression
+                          ->  HashAggregate
+                                Group Key: l_quantity
+                                ->  Seq Scan on lineitem_290001 lineitem
+SET enable_hashagg TO on;

src/test/regress/expected/multi_explain_0.out

@@ -657,3 +657,23 @@ Custom Scan (Citus Router)
         ->  Seq Scan on explain_table_570001 explain_table
               Filter: (id = 1)
 ROLLBACK;
+-- Test disable hash aggregate
+SET enable_hashagg TO off;
+EXPLAIN (COSTS FALSE, FORMAT TEXT)
+	SELECT l_quantity, count(*) count_quantity FROM lineitem
+	GROUP BY l_quantity ORDER BY count_quantity, l_quantity;
+Sort
+  Sort Key: COALESCE((pg_catalog.sum((COALESCE((pg_catalog.sum(remote_scan.count_quantity))::bigint, '0'::bigint))))::bigint, '0'::bigint), remote_scan.l_quantity
+  ->  GroupAggregate
+        Group Key: remote_scan.l_quantity
+        ->  Sort
+              Sort Key: remote_scan.l_quantity
+              ->  Custom Scan (Citus Real-Time)
+                    Task Count: 8
+                    Tasks Shown: One of 8
+                    ->  Task
+                          Node: host=localhost port=57637 dbname=regression
+                          ->  HashAggregate
+                                Group Key: l_quantity
+                                ->  Seq Scan on lineitem_290001 lineitem
+SET enable_hashagg TO on;

src/test/regress/sql/multi_explain.sql

@@ -247,3 +247,11 @@ ALTER TABLE explain_table ADD COLUMN value int;
 EXPLAIN (COSTS FALSE) SELECT value FROM explain_table WHERE id = 1;
 
 ROLLBACK;
+
+-- Test disable hash aggregate
+SET enable_hashagg TO off;
+EXPLAIN (COSTS FALSE, FORMAT TEXT)
+	SELECT l_quantity, count(*) count_quantity FROM lineitem
+	GROUP BY l_quantity ORDER BY count_quantity, l_quantity;
+
+SET enable_hashagg TO on;