Fix overeager pushdown of HAVING clauses when grouping sets are used.

In 61444bfb we started to allow HAVING clauses to be fully pushed down
into WHERE, even when grouping sets are in use. That turns out not to
work correctly, because grouping sets can "produce" NULLs, meaning that
filtering in WHERE and HAVING can have different results, even when no
aggregates or volatile functions are involved.

Instead only allow pushdown of empty grouping sets.

It'd be nice to do better, but the exact mechanics of deciding which
cases are safe are still being debated. It's important to give correct
results till we find a good solution, and such a solution might not be
appropriate for backpatching anyway.

Bug: #13863
Reported-By: 'wrb'
Diagnosed-By: Dean Rasheed
Author: Andrew Gierth
Reviewed-By: Dean Rasheed and Andres Freund
Discussion: 20160113183558.12989.56904@wrigleys.postgresql.org
Backpatch: 9.5, where grouping sets were introduced

src/test/regress/expected/groupingsets.out

@@ -607,6 +607,60 @@ having exists (select 1 from onek b where sum(distinct a.four) = b.four);
    9 |   3
 (25 rows)
 
+-- Tests around pushdown of HAVING clauses, partially testing against previous bugs
+select a,count(*) from gstest2 group by rollup(a) order by a;
+ a | count 
+---+-------
+ 1 |     8
+ 2 |     1
+   |     9
+(3 rows)
+
+select a,count(*) from gstest2 group by rollup(a) having a is distinct from 1 order by a;
+ a | count 
+---+-------
+ 2 |     1
+   |     9
+(2 rows)
+
+explain (costs off)
+  select a,count(*) from gstest2 group by rollup(a) having a is distinct from 1 order by a;
+            QUERY PLAN            
+----------------------------------
+ GroupAggregate
+   Group Key: a
+   Group Key: ()
+   Filter: (a IS DISTINCT FROM 1)
+   ->  Sort
+         Sort Key: a
+         ->  Seq Scan on gstest2
+(7 rows)
+
+select v.c, (select count(*) from gstest2 group by () having v.c)
+  from (values (false),(true)) v(c) order by v.c;
+ c | count 
+---+-------
+ f |      
+ t |     9
+(2 rows)
+
+explain (costs off)
+  select v.c, (select count(*) from gstest2 group by () having v.c)
+    from (values (false),(true)) v(c) order by v.c;
+                        QUERY PLAN                         
+-----------------------------------------------------------
+ Sort
+   Sort Key: "*VALUES*".column1
+   ->  Values Scan on "*VALUES*"
+         SubPlan 1
+           ->  Aggregate
+                 Group Key: ()
+                 Filter: "*VALUES*".column1
+                 ->  Result
+                       One-Time Filter: "*VALUES*".column1
+                       ->  Seq Scan on gstest2
+(10 rows)
+
 -- HAVING with GROUPING queries
 select ten, grouping(ten) from onek
 group by grouping sets(ten) having grouping(ten) >= 0

src/test/regress/sql/groupingsets.sql

@@ -183,6 +183,18 @@ select ten, sum(distinct four) from onek a
 group by grouping sets((ten,four),(ten))
 having exists (select 1 from onek b where sum(distinct a.four) = b.four);
 
+-- Tests around pushdown of HAVING clauses, partially testing against previous bugs
+select a,count(*) from gstest2 group by rollup(a) order by a;
+select a,count(*) from gstest2 group by rollup(a) having a is distinct from 1 order by a;
+explain (costs off)
+  select a,count(*) from gstest2 group by rollup(a) having a is distinct from 1 order by a;
+
+select v.c, (select count(*) from gstest2 group by () having v.c)
+  from (values (false),(true)) v(c) order by v.c;
+explain (costs off)
+  select v.c, (select count(*) from gstest2 group by () having v.c)
+    from (values (false),(true)) v(c) order by v.c;
+
 -- HAVING with GROUPING queries
 select ten, grouping(ten) from onek
 group by grouping sets(ten) having grouping(ten) >= 0