[HACKERS] GatherMerge misses to push target list

During my recent work on costing of parallel paths [1], I noticed that
we are missing to push target list below GatherMerge in some simple
cases like below.

Test prepration
---------------------
create or replace function simple_func(var1 integer) returns integer
as $$
begin
        return var1 + 10;
end;
$$ language plpgsql PARALLEL SAFE;

create table t1(c1 int, c2 char(5));
insert into t1 values(generate_series(1,500000), 'aaa');
set parallel_setup_cost=0;
set parallel_tuple_cost=0;
set min_parallel_table_scan_size=0;
set max_parallel_workers_per_gather=4;
set cpu_operator_cost=0; set min_parallel_index_scan_size=0;

Case-1
-------------
postgres=# explain (costs off, verbose) select c1,simple_func(c1) from
t1 where c1 < 10000 order by c1;
                     QUERY PLAN
-----------------------------------------------------
 Gather Merge
   Output: c1, simple_func(c1)
   Workers Planned: 4
   ->  Parallel Index Scan using idx_t1 on public.t1
         Output: c1
         Index Cond: (t1.c1 < 10000)
(6 rows)

In the above case, I don't see any reason why we can't push the target
list expression (simple_func(c1)) down to workers.

Case-2
----------
set enable_indexonlyscan=off;
set enable_indexscan=off;
postgres=# explain (costs off, verbose) select c1,simple_func(c1) from
t1 where c1 < 10000 order by c1;
                     QUERY PLAN
----------------------------------------------------
 Gather Merge
   Output: c1, simple_func(c1)
   Workers Planned: 4
   ->  Sort
         Output: c1
         Sort Key: t1.c1
         ->  Parallel Bitmap Heap Scan on public.t1
               Output: c1
               Recheck Cond: (t1.c1 < 10000)
               ->  Bitmap Index Scan on idx_t1
                     Index Cond: (t1.c1 < 10000)
(11 rows)

It is different from above case (Case-1) because sort node can't
project, but I think adding a Result node on top of it can help to
project and serve our purpose.

The attached patch allows pushing the target list expression in both
the cases. I think we should handle GatherMerge case in
apply_projection_to_path similar to Gather so that target list can be
pushed.  Another problem was during the creation of ordered paths
where we don't allow target expressions to be pushed below gather
merge.

Plans after patch:

Case-1
----------
postgres=# explain (costs off, verbose) select c1,simple_func(c1) from
t1 where c1 < 10000 order by c1;
                        QUERY PLAN
----------------------------------------------------------
 Gather Merge
   Output: c1, (simple_func(c1))
   Workers Planned: 4
   ->  Parallel Index Only Scan using idx_t1 on public.t1
         Output: c1, simple_func(c1)
         Index Cond: (t1.c1 < 10000)
(6 rows)

Case-2
-----------
postgres=# explain (costs off, verbose) select c1,simple_func(c1) from
t1 where c1 < 10000 order by c1;
                        QUERY PLAN
----------------------------------------------------------
 Gather Merge
   Output: c1, (simple_func(c1))
   Workers Planned: 4
   ->  Result
         Output: c1, simple_func(c1)
         ->  Sort
               Output: c1
               Sort Key: t1.c1
               ->  Parallel Bitmap Heap Scan on public.t1
                     Output: c1
                     Recheck Cond: (t1.c1 < 10000)
                     ->  Bitmap Index Scan on idx_t1
                           Index Cond: (t1.c1 < 10000)
(13 rows)

Note, that simple_func() is pushed down to workers in both the cases.

Thoughts?

Note - If we agree on the problems and fix, then I can add regression
tests to cover above cases in the patch.

[1] - https://www.postgresql.org/message-id/CAA4eK1Ji_0pgrjFotAyvvfxGikxJQEKcxD863VQ-xYtfQBy0uQ%40mail.gmail.com

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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