Avoid pushing quals down into sub-queries that have grouping sets.

The trouble with doing this is that an apparently-constant subquery
output column isn't really constant if it is a grouping column that
appears in only some of the grouping sets.  A qual using such a
column would be subject to incorrect const-folding after push-down,
as seen in bug #16585 from Paul Sivash.

To fix, just disable qual pushdown altogether if the sub-query has
nonempty groupingSets.  While we could imagine far less restrictive
solutions, there is not much point in working harder right now,
because subquery_planner() won't move HAVING clauses to WHERE within
such a subquery.  If the qual stays in HAVING it's not going to be
a lot more useful than if we'd kept it at the outer level.

Having said that, this restriction could be removed if we used a
parsetree representation that distinguished such outputs from actual
constants, which is something I hope to do in future.  Hence, make
the patch a minimal addition rather than integrating it more tightly
(e.g. by renumbering the existing items in subquery_is_pushdown_safe's
comment).

Back-patch to 9.5 where grouping sets were introduced.

Discussion: https://postgr.es/m/16585-9d8c340d23ade8c1@postgresql.org

src/test/regress/expected/groupingsets.out

@@ -434,6 +434,38 @@ select x, not x as not_x, q2 from
    |       |  4567890123456789
 (5 rows)
 
+-- check qual push-down rules for a subquery with grouping sets
+explain (verbose, costs off)
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+                 QUERY PLAN                 
+--------------------------------------------
+ Subquery Scan on ss
+   Output: ss.x, ss.q1, ss.sum
+   Filter: ((ss.x = 1) AND (ss.q1 = 123))
+   ->  GroupAggregate
+         Output: (1), i1.q1, sum(i1.q2)
+         Group Key: 1
+         Sort Key: i1.q1
+           Group Key: i1.q1
+         ->  Seq Scan on public.int8_tbl i1
+               Output: 1, i1.q1, i1.q2
+(10 rows)
+
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+ x | q1 | sum 
+---+----+-----
+(0 rows)
+
 -- simple rescan tests
 select a, b, sum(v.x)
   from (values (1),(2)) v(x), gstest_data(v.x)

src/test/regress/sql/groupingsets.sql

@@ -172,6 +172,22 @@ select x, not x as not_x, q2 from
   group by grouping sets(x, q2)
   order by x, q2;
 
+-- check qual push-down rules for a subquery with grouping sets
+explain (verbose, costs off)
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+
 -- simple rescan tests
 
 select a, b, sum(v.x)