BUG #15679: Partial HASH index takes too much space

The following bug has been logged on the website:

Bug reference: 15679
Logged by: Ortwin Gentz
Email address: gentz(at)futuretap(dot)com
PostgreSQL version: 10.6
Operating system: Amazon RDS
Description:

I created a partial HASH index for a sparsely populated column:

CREATE INDEX partial_hash ON mytable USING HASH(my_id) WHERE my_ID IS NOT
NULL;

Even though the my_id VARCHAR(255) column is populated (NON NULL) only for a
few hundred records, the index takes 256 MB of space (for a table with > 10m
records). Also, it doesn't make a difference if the index is created as a
partial index ("WHERE my_ID IS NOT NULL") or as a full index.

In contrast to that, a BTREE index differs considerably in space for full
and partial:

CREATE INDEX full_btree ON mytable (my_id); # 543 MB
CREATE INDEX partial_btree ON mytable (my_id) WHERE my_ID IS NOT NULL; # 16
KB

See also this StackExchange conversation where user jjanes considers the
behavior a bug in the hash index code:
https://dba.stackexchange.com/a/231660/25337