BUG #16840: Rows not found in table partitioned by hash when not all partitions exists

The following bug has been logged on the website:

Bug reference:      16840
Logged by:          Michał Albrycht
Email address:      michalalbrycht@gmail.com
PostgreSQL version: 13.1
Operating system:   Ubuntu 18
Description:

Lets create a table partitioned by hash:

CREATE TABLE dir (
   id SERIAL,
   volume_id BIGINT,
   path TEXT
) PARTITION BY HASH (volume_id);


Now let's create just one partition:

CREATE TABLE  dir_part_2 PARTITION OF dir FOR VALUES WITH (modulus 3,
remainder 2);

Insert sample value:

INSERT INTO dir (volume_id, path) VALUES
    (1, 'abc'),
    (1, 'def'),
    (1, 'ghi');
    
Queries to verify db state:
SELECT * FROM dir WHERE volume_id=1 AND path='abc'; -- returns 1 row - OK
SELECT * FROM dir WHERE volume_id=1 AND (path='abc' OR path='def')  --
returns 0 rows - NOT OK!
SELECT * FROM dir_part_2 WHERE volume_id=1 AND (path='abc' or path='def') --
returns 2 rows - OK

Now lets add missing partitions:
CREATE TABLE  dir_part_0 PARTITION OF dir FOR VALUES WITH (modulus 3,
remainder 0);
CREATE TABLE  dir_part_1 PARTITION OF dir FOR VALUES WITH (modulus 3,
remainder 1);

And run problematic query one more time:
SELECT * FROM dir WHERE volume_id=1 AND (path='abc' OR path='def')  --
returns 2 rows - OK


So Postgres silently fails to find a rows, and does not throw any error when
not all hash partitions are present.