diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 1a0d3a885f9..1a9fd829008 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -6326,7 +6326,6 @@ make_setop(SetOpCmd cmd, SetOpStrategy strategy, Plan *lefttree,
 	 * convert SortGroupClause list into arrays of attr indexes and equality
 	 * operators, as wanted by executor
 	 */
-	Assert(numCols > 0);
 	dupColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numCols);
 	dupOperators = (Oid *) palloc(sizeof(Oid) * numCols);
 
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index a24e8acfa6c..f87849ea476 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -711,10 +711,6 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
-	if (groupList == NIL)
-		return path;
-
 	/*
 	 * Estimate number of distinct groups that we'll need hashtable entries
 	 * for; this is the size of the left-hand input for EXCEPT, or the smaller
@@ -741,7 +737,7 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
 								   dNumGroups, dNumOutputRows,
 								   (op->op == SETOP_INTERSECT) ? "INTERSECT" : "EXCEPT");
 
-	if (!use_hash)
+	if (groupList && !use_hash)
 		path = (Path *) create_sort_path(root,
 										 result_rel,
 										 path,
@@ -864,10 +860,6 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
-	if (groupList == NIL)
-		return path;
-
 	/*
 	 * XXX for the moment, take the number of distinct groups as equal to the
 	 * total input size, ie, the worst case.  This is too conservative, but we
@@ -898,13 +890,15 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
 	else
 	{
 		/* Sort and Unique */
-		path = (Path *) create_sort_path(root,
-										 result_rel,
-										 path,
-										 make_pathkeys_for_sortclauses(root,
-																	   groupList,
-																	   tlist),
-										 -1.0);
+		if (groupList)
+			path = (Path *)
+				create_sort_path(root,
+								 result_rel,
+								 path,
+								 make_pathkeys_for_sortclauses(root,
+															   groupList,
+															   tlist),
+								 -1.0);
 		/* We have to manually jam the right tlist into the path; ick */
 		path->pathtarget = create_pathtarget(root, tlist);
 		path = (Path *) create_upper_unique_path(root,
diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index ee26b163f7d..92d427a690f 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -552,6 +552,121 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
  4567890123456789 | -4567890123456789
 (5 rows)
 
+--
+-- Check behavior with empty select list (allowed since 9.4)
+--
+select union select;
+--
+(1 row)
+
+select intersect select;
+--
+(1 row)
+
+select except select;
+--
+(0 rows)
+
+-- check hashed implementation
+set enable_hashagg = true;
+set enable_sort = false;
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ HashAggregate
+   ->  Append
+         ->  Function Scan on generate_series
+         ->  Function Scan on generate_series generate_series_1
+(4 rows)
+
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ HashSetOp Intersect
+   ->  Append
+         ->  Subquery Scan on "*SELECT* 1"
+               ->  Function Scan on generate_series
+         ->  Subquery Scan on "*SELECT* 2"
+               ->  Function Scan on generate_series generate_series_1
+(6 rows)
+
+select from generate_series(1,5) union select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) union all select from generate_series(1,3);
+--
+(8 rows)
+
+select from generate_series(1,5) intersect select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+--
+(3 rows)
+
+select from generate_series(1,5) except select from generate_series(1,3);
+--
+(0 rows)
+
+select from generate_series(1,5) except all select from generate_series(1,3);
+--
+(2 rows)
+
+-- check sorted implementation
+set enable_hashagg = false;
+set enable_sort = true;
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ Unique
+   ->  Append
+         ->  Function Scan on generate_series
+         ->  Function Scan on generate_series generate_series_1
+(4 rows)
+
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ SetOp Intersect
+   ->  Append
+         ->  Subquery Scan on "*SELECT* 1"
+               ->  Function Scan on generate_series
+         ->  Subquery Scan on "*SELECT* 2"
+               ->  Function Scan on generate_series generate_series_1
+(6 rows)
+
+select from generate_series(1,5) union select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) union all select from generate_series(1,3);
+--
+(8 rows)
+
+select from generate_series(1,5) intersect select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+--
+(3 rows)
+
+select from generate_series(1,5) except select from generate_series(1,3);
+--
+(0 rows)
+
+select from generate_series(1,5) except all select from generate_series(1,3);
+--
+(2 rows)
+
+reset enable_hashagg;
+reset enable_sort;
 --
 -- Check handling of a case with unknown constants.  We don't guarantee
 -- an undecorated constant will work in all cases, but historically this
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index c0317cccb4b..eed7c8d34be 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -190,6 +190,49 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
 
 (((((select * from int8_tbl)))));
 
+--
+-- Check behavior with empty select list (allowed since 9.4)
+--
+
+select union select;
+select intersect select;
+select except select;
+
+-- check hashed implementation
+set enable_hashagg = true;
+set enable_sort = false;
+
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+
+select from generate_series(1,5) union select from generate_series(1,3);
+select from generate_series(1,5) union all select from generate_series(1,3);
+select from generate_series(1,5) intersect select from generate_series(1,3);
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+select from generate_series(1,5) except select from generate_series(1,3);
+select from generate_series(1,5) except all select from generate_series(1,3);
+
+-- check sorted implementation
+set enable_hashagg = false;
+set enable_sort = true;
+
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+
+select from generate_series(1,5) union select from generate_series(1,3);
+select from generate_series(1,5) union all select from generate_series(1,3);
+select from generate_series(1,5) intersect select from generate_series(1,3);
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+select from generate_series(1,5) except select from generate_series(1,3);
+select from generate_series(1,5) except all select from generate_series(1,3);
+
+reset enable_hashagg;
+reset enable_sort;
+
 --
 -- Check handling of a case with unknown constants.  We don't guarantee
 -- an undecorated constant will work in all cases, but historically this