Fix pushdown of degenerate HAVING clauses

67a54b9e8 taught the planner to push down HAVING clauses even when
grouping sets are present, as long as the clause does not reference
any columns that are nullable by the grouping sets.  However, there
was an oversight: if any empty grouping sets are present, the
aggregation node can produce a row that did not come from the input,
and pushing down a HAVING clause in this case may cause us to fail to
filter out that row.

Currently, non-degenerate HAVING clauses are not pushed down when
empty grouping sets are present, since the empty grouping sets would
nullify the vars they reference.  However, degenerate (variable-free)
HAVING clauses are not subject to this restriction and may be
incorrectly pushed down.

To fix, explicitly check for the presence of empty grouping sets and
retain degenerate clauses in HAVING when they are present.  This
ensures that we don't emit a bogus aggregated row.  A copy of each
such clause is also put in WHERE so that query_planner() can use it in
a gating Result node.

To facilitate this check, this patch expands the groupingSets tree of
the query to a flat list of grouping sets before applying the HAVING
pushdown optimization.  This does not add any additional planning
overhead, since we need to do this expansion anyway.

In passing, make a small tweak to preprocess_grouping_sets() by
reordering its initial operations a bit.

Backpatch to v18, where this issue was introduced.

Reported-by: Yuhang Qiu <iamqyh@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/0879D9C9-7FE2-4A20-9593-B23F7A0B5290@gmail.com
Backpatch-through: 18

src/test/regress/expected/groupingsets.out

@@ -890,7 +890,8 @@ explain (costs off)
                        ->  Seq Scan on gstest2
 (10 rows)
 
--- test pushdown of HAVING clause that does not reference any columns that are nullable by grouping sets
+-- test pushdown of non-degenerate HAVING clause that does not reference any
+-- columns that are nullable by grouping sets
 explain (costs off)
 select a, b, count(*) from gstest2 group by grouping sets ((a, b), (a)) having a > 1 and b > 1;
            QUERY PLAN            
@@ -911,6 +912,85 @@ select a, b, count(*) from gstest2 group by grouping sets ((a, b), (a)) having a
  2 | 2 |     1
 (1 row)
 
+explain (costs off)
+select a, b, count(*) from gstest2 group by rollup(a), b having b > 1;
+           QUERY PLAN            
+---------------------------------
+ GroupAggregate
+   Group Key: b, a
+   Group Key: b
+   ->  Sort
+         Sort Key: b, a
+         ->  Seq Scan on gstest2
+               Filter: (b > 1)
+(7 rows)
+
+select a, b, count(*) from gstest2 group by rollup(a), b having b > 1;
+ a | b | count 
+---+---+-------
+ 1 | 2 |     1
+ 2 | 2 |     1
+   | 2 |     2
+(3 rows)
+
+-- test pushdown of degenerate HAVING clause
+explain (costs off)
+select count(*) from gstest2 group by grouping sets (()) having false;
+            QUERY PLAN             
+-----------------------------------
+ Aggregate
+   Group Key: ()
+   Filter: false
+   ->  Result
+         Replaces: Scan on gstest2
+         One-Time Filter: false
+(6 rows)
+
+select count(*) from gstest2 group by grouping sets (()) having false;
+ count 
+-------
+(0 rows)
+
+explain (costs off)
+select a, count(*) from gstest2 group by grouping sets ((a), ()) having false;
+               QUERY PLAN                
+-----------------------------------------
+ GroupAggregate
+   Group Key: a
+   Group Key: ()
+   Filter: false
+   ->  Sort
+         Sort Key: a
+         ->  Result
+               Replaces: Scan on gstest2
+               One-Time Filter: false
+(9 rows)
+
+select a, count(*) from gstest2 group by grouping sets ((a), ()) having false;
+ a | count 
+---+-------
+(0 rows)
+
+explain (costs off)
+select a, b, count(*) from gstest2 group by grouping sets ((a), (b)) having false;
+               QUERY PLAN                
+-----------------------------------------
+ GroupAggregate
+   Group Key: a
+   Sort Key: b
+     Group Key: b
+   ->  Sort
+         Sort Key: a
+         ->  Result
+               Replaces: Scan on gstest2
+               One-Time Filter: false
+(9 rows)
+
+select a, b, count(*) from gstest2 group by grouping sets ((a), (b)) having false;
+ a | b | count 
+---+---+-------
+(0 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

@@ -290,11 +290,29 @@ 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;
 
--- test pushdown of HAVING clause that does not reference any columns that are nullable by grouping sets
+-- test pushdown of non-degenerate HAVING clause that does not reference any
+-- columns that are nullable by grouping sets
 explain (costs off)
 select a, b, count(*) from gstest2 group by grouping sets ((a, b), (a)) having a > 1 and b > 1;
 select a, b, count(*) from gstest2 group by grouping sets ((a, b), (a)) having a > 1 and b > 1;
 
+explain (costs off)
+select a, b, count(*) from gstest2 group by rollup(a), b having b > 1;
+select a, b, count(*) from gstest2 group by rollup(a), b having b > 1;
+
+-- test pushdown of degenerate HAVING clause
+explain (costs off)
+select count(*) from gstest2 group by grouping sets (()) having false;
+select count(*) from gstest2 group by grouping sets (()) having false;
+
+explain (costs off)
+select a, count(*) from gstest2 group by grouping sets ((a), ()) having false;
+select a, count(*) from gstest2 group by grouping sets ((a), ()) having false;
+
+explain (costs off)
+select a, b, count(*) from gstest2 group by grouping sets ((a), (b)) having false;
+select a, b, count(*) from gstest2 group by grouping sets ((a), (b)) having false;
+
 -- HAVING with GROUPING queries
 select ten, grouping(ten) from onek
 group by grouping sets(ten) having grouping(ten) >= 0