Multi-level hierarchy with parallel append can lead to an extra subplan.

Hi,

I am getting extra subplan when using parallel append with multi-level hierarchy,  leading to data corruption.
Please see below test case.

-- set below parameters to encourage use of parallel plans
SET parallel_setup_cost=0;
SET parallel_tuple_cost=0;
SET min_parallel_table_scan_size=0;
SET max_parallel_workers_per_gather=4;

--create below data set
CREATE TABLE RM38941_inherit (c1 INTEGER PRIMARY KEY,c2 INTEGER,c3 CHAR(10));
INSERT INTO RM38941_inherit SELECT i, i % 125, to_char(i % 4, 'FM0000') FROM generate_series(0, 499,2) i;

CREATE TABLE RM38941_inherit_t1 () INHERITS (RM38941_inherit);
INSERT INTO RM38941_inherit_t1 SELECT i, i % 125, to_char(i % 4, 'FM0000') FROM generate_series(0, 499,3) i;

CREATE TABLE RM38941_inherit_t2 () INHERITS (RM38941_inherit);
INSERT INTO RM38941_inherit_t2 SELECT i, i % 125, to_char(i % 4, 'FM0000') FROM generate_series(0, 499,5) i;

CREATE TABLE RM38941_union_t1 (c1 INTEGER PRIMARY KEY,c2 INTEGER,c3 CHAR(10));
INSERT INTO RM38941_union_t1 SELECT i, i % 125, to_char(i % 4, 'FM0000') FROM generate_series(0, 499,2) i;

CREATE TABLE RM38941_union_t2 (c1 INTEGER PRIMARY KEY,c2 INTEGER,c3 CHAR(10));
INSERT INTO RM38941_union_t2 SELECT i, i % 125, to_char(i % 4, 'FM0000') FROM generate_series(0, 499,4) i;

ALTER TABLE RM38941_union_t1 SET (parallel_workers = 0);
ALTER TABLE RM38941_inherit_t1 SET (parallel_workers = 0);

--with parallel_append
SET enable_parallel_append=on;
postgres=# EXPLAIN (COSTS OFF)
postgres-# SELECT AVG(c1),SUM(c2) FROM (SELECT c1,c2 FROM RM38941_union_t2 UNION ALL SELECT c1,c2 FROM RM38941_inherit UNION ALL SELECT c1,c2 FROM RM38941_union_t1)UA;
                              QUERY PLAN                              
-----------------------------------------------------------------------
 Finalize Aggregate
   ->  Gather
         Workers Planned: 3
         ->  Partial Aggregate
               ->  Parallel Append
                     ->  Seq Scan on rm38941_inherit_t1
                     ->  Seq Scan on rm38941_union_t1
                     ->  Parallel Seq Scan on rm38941_union_t2
                     ->  Parallel Seq Scan on rm38941_inherit
                     ->  Parallel Seq Scan on rm38941_inherit_t2
                     ->  Parallel Append
                           ->  Seq Scan on rm38941_inherit_t1
                           ->  Parallel Seq Scan on rm38941_inherit
                           ->  Parallel Seq Scan on rm38941_inherit_t2
(14 rows)

postgres=# SELECT AVG(c1),SUM(c2) FROM (SELECT c1,c2 FROM RM38941_union_t2 UNION ALL SELECT c1,c2 FROM RM38941_inherit UNION ALL SELECT c1,c2 FROM RM38941_union_t1)UA;
         avg          |  sum 
----------------------+-------
 248.6983676366217175 | 86916
(1 row)


--without parallel_append
SET enable_parallel_append=off;
postgres=# EXPLAIN (COSTS OFF)
postgres-# SELECT AVG(c1),SUM(c2) FROM (SELECT c1,c2 FROM RM38941_union_t2 UNION ALL SELECT c1,c2 FROM RM38941_inherit UNION ALL SELECT c1,c2 FROM RM38941_union_t1)UA;
                 QUERY PLAN                
--------------------------------------------
 Aggregate
   ->  Append
         ->  Seq Scan on rm38941_union_t2
         ->  Seq Scan on rm38941_inherit
         ->  Seq Scan on rm38941_inherit_t1
         ->  Seq Scan on rm38941_inherit_t2
         ->  Seq Scan on rm38941_union_t1
(7 rows)

postgres=# SELECT AVG(c1),SUM(c2) FROM (SELECT c1,c2 FROM RM38941_union_t2 UNION ALL SELECT c1,c2 FROM RM38941_inherit UNION ALL SELECT c1,c2 FROM RM38941_union_t1)UA;
         avg          |  sum 
----------------------+-------
 248.6917040358744395 | 55083
(1 row)