BUG #15093: hash partition exector plan is error!!!!

The following bug has been logged on the website:

Bug reference:      15093
Logged by:          jiang fei
Email address:      weiboyiyou@163.com
PostgreSQL version: Unsupported/Unknown
Operating system:   CentOS release 6.8
Description:

I create a table like this:
test=# \d+ orders
                                   Table "public.orders"
  Column  |  Type  | Collation | Nullable | Default | Storage  | Stats
target | Description 
----------+--------+-----------+----------+---------+----------+--------------+-------------
 order_id | bigint |           | not null |         | plain    |
 | 
 cust_id  | bigint |           | not null |         | plain    |
 | 
 status   | text   |           |          |         | extended |
 | 
Partition key: HASH (order_id)
Partitions: orders_p1 FOR VALUES WITH (modulus 4, remainder 0),
            orders_p2 FOR VALUES WITH (modulus 4, remainder 1),
            orders_p3 FOR VALUES WITH (modulus 4, remainder 2),
            orders_p4 FOR VALUES WITH (modulus 4, remainder 3)


but the exector plan is:
test=# explain select * from orders where order_id = 1;
                           QUERY PLAN                            
-----------------------------------------------------------------
 Append  (cost=0.00..93.60 rows=20 width=48)
   ->  Seq Scan on orders_p1  (cost=0.00..23.38 rows=5 width=48)
         Filter: (order_id = 1)
   ->  Seq Scan on orders_p2  (cost=0.00..23.38 rows=5 width=48)
         Filter: (order_id = 1)
   ->  Seq Scan on orders_p3  (cost=0.00..23.38 rows=5 width=48)
         Filter: (order_id = 1)
   ->  Seq Scan on orders_p4  (cost=0.00..23.38 rows=5 width=48)
         Filter: (order_id = 1)