diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index e90e1d6..5a48a02 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -1734,7 +1734,7 @@ findTargetlistEntrySQL99(ParseState *pstate, Node *node, List **tlist,
  * Inside a grouping set (ROLLUP, CUBE, or GROUPING SETS), we expect the
  * content to be nested no more than 2 deep: i.e. ROLLUP((a,b),(c,d)) is
  * ok, but ROLLUP((a,(b,c)),d) is flattened to ((a,b,c),d), which we then
- * normalize to ((a,b,c),(d)).
+ * (later) normalize to ((a,b,c),(d)).
  *
  * CUBE or ROLLUP can be nested inside GROUPING SETS (but not the reverse),
  * and we leave that alone if we find it. But if we see GROUPING SETS inside
@@ -1803,9 +1803,16 @@ flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
 
 				foreach(l2, gset->content)
 				{
-					Node	   *n2 = flatten_grouping_sets(lfirst(l2), false, NULL);
+					Node	   *n1 = lfirst(l2);
+					Node	   *n2 = flatten_grouping_sets(n1, false, NULL);
 
-					result_set = lappend(result_set, n2);
+					if (IsA(n1, GroupingSet) &&
+						((GroupingSet *)n1)->kind == GROUPING_SET_SETS)
+					{
+						result_set = list_concat(result_set, (List *) n2);
+					}
+					else
+						result_set = lappend(result_set, n2);
 				}
 
 				/*
diff --git a/src/test/regress/expected/groupingsets.out b/src/test/regress/expected/groupingsets.out
index 842c2ae..ff3ba9b 100644
--- a/src/test/regress/expected/groupingsets.out
+++ b/src/test/regress/expected/groupingsets.out
@@ -145,6 +145,127 @@ select a, b, sum(c), sum(sum(c)) over (order by a,b) as rsum
    |   |  12 |   36
 (6 rows)
 
+-- nesting with grouping sets
+select sum(c) from gstest2
+  group by grouping sets((), grouping sets((), grouping sets(())))
+  order by 1 desc;
+ sum 
+-----
+  12
+  12
+  12
+(3 rows)
+
+select sum(c) from gstest2
+  group by grouping sets((), grouping sets((), grouping sets(((a, b)))))
+  order by 1 desc;
+ sum 
+-----
+  12
+  12
+   8
+   2
+   2
+(5 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(rollup(c), grouping sets(cube(c))))
+  order by 1 desc;
+ sum 
+-----
+  12
+  12
+   6
+   6
+   6
+   6
+(6 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(a, grouping sets(a, cube(b)))
+  order by 1 desc;
+ sum 
+-----
+  12
+  10
+  10
+   8
+   4
+   2
+   2
+(7 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(grouping sets((a, (b))))
+  order by 1 desc;
+ sum 
+-----
+   8
+   2
+   2
+(3 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(grouping sets((a, b)))
+  order by 1 desc;
+ sum 
+-----
+   8
+   2
+   2
+(3 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(a, grouping sets(a), a))
+  order by 1 desc;
+ sum 
+-----
+  10
+  10
+  10
+   2
+   2
+   2
+(6 rows)
+
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(a, grouping sets(a, grouping sets(a), ((a)), a, grouping sets(a), (a)), a))
+  order by 1 desc;
+ sum 
+-----
+  10
+  10
+  10
+  10
+  10
+  10
+  10
+  10
+   2
+   2
+   2
+   2
+   2
+   2
+   2
+   2
+(16 rows)
+
+select sum(c) from gstest2
+  group by grouping sets((a,(a,b)), grouping sets((a,(a,b)),a))
+  order by 1 desc;
+ sum 
+-----
+  10
+   8
+   8
+   2
+   2
+   2
+   2
+   2
+(8 rows)
+
 -- empty input: first is 0 rows, second 1, third 3 etc.
 select a, b, sum(v), count(*) from gstest_empty group by grouping sets ((a,b),a);
  a | b | sum | count 
diff --git a/src/test/regress/sql/groupingsets.sql b/src/test/regress/sql/groupingsets.sql
index 0bffb85..d886fae 100644
--- a/src/test/regress/sql/groupingsets.sql
+++ b/src/test/regress/sql/groupingsets.sql
@@ -73,6 +73,35 @@ select grouping(a), a, array_agg(b),
 select a, b, sum(c), sum(sum(c)) over (order by a,b) as rsum
   from gstest2 group by rollup (a,b) order by rsum, a, b;
 
+-- nesting with grouping sets
+select sum(c) from gstest2
+  group by grouping sets((), grouping sets((), grouping sets(())))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets((), grouping sets((), grouping sets(((a, b)))))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(rollup(c), grouping sets(cube(c))))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(a, grouping sets(a, cube(b)))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(grouping sets((a, (b))))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(grouping sets((a, b)))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(a, grouping sets(a), a))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets(grouping sets(a, grouping sets(a, grouping sets(a), ((a)), a, grouping sets(a), (a)), a))
+  order by 1 desc;
+select sum(c) from gstest2
+  group by grouping sets((a,(a,b)), grouping sets((a,(a,b)),a))
+  order by 1 desc;
+
 -- empty input: first is 0 rows, second 1, third 3 etc.
 select a, b, sum(v), count(*) from gstest_empty group by grouping sets ((a,b),a);
 select a, b, sum(v), count(*) from gstest_empty group by grouping sets ((a,b),());