Fix flattening of nested grouping sets.

Previously nested grouping set specifications accidentally weren't
flattened, but instead contained the nested specification as a element
in the outer list.

Fix this by, as actually documented in comments, concatenating the
nested set specification into the outer one. Also add tests to prevent
this from breaking again.

Author: Andrew Gierth, with tests from Jeevan Chalke
Reported-By: Jeevan Chalke
Discussion: CAM2+6=V5YvuxB+EyN4iH=GbD-XTA435TCNvnDFSD--YvXs+pww@mail.gmail.com
Backpatch: 9.5, where grouping sets were introduced

src/backend/parser/parse_clause.c

@@ -1812,7 +1812,7 @@ 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
@@ -1881,9 +1881,16 @@ 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);
 				}
 
 				/*

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 

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),());