BUG #17594: conditional hash indexes size (hash index ignore WHERE condition during CREATE INDEX?)

The following bug has been logged on the website:

Bug reference: 17594
Logged by: Maxim Boguk
Email address: maxim(dot)boguk(at)gmail(dot)com
PostgreSQL version: 14.4
Operating system: Linux
Description:

Hi,

I found very weird situation with size of highly selective partial hash
indexes (they has almost same size as full table hash index).
Test case:

create table test_1000000 as select id from generate_series(1, 1000000) as
g(id);
create index test_1000000_hash_full_idx on test_1000000 using hash(id);
create index test_1000000_hash_partial_idx on test_1000000 using hash(id)
where id<11;
create table test_10 as select id from generate_series(1, 10) as g(id);
create index test_10_hash_full on test_10 using hash(id);

\di+ test_1*
List of relations
Schema | Name | Type | Owner | Table |
Persistence | Access method | Size | Description
--------+-------------------------------+-------+--------+--------------+-------------+---------------+-------+-------------
public | test_1000000_hash_full_idx | index | mboguk | test_1000000 |
permanent | hash | 32 MB |
public | test_1000000_hash_partial_idx | index | mboguk | test_1000000 |
permanent | hash | 28 MB |
public | test_10_hash_full | index | mboguk | test_10 |
permanent | hash | 80 kB |

Expected: the test_1000000_hash_partial_idx and test_10_hash_full indexes
should have same or at least close size because they index same amount of
rows (10):

select count(*) from test_10;
count
-------
10

select count(*) from test_1000000 where id<11;
count
-------
10