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(at)163(dot)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)