CentOS 7
$ rpm -q postgresql10
postgresql10-10.6-1PGDG.rhel7.x86_64
SQL script for psql:
\set table_size 1000000
begin;
create table gender (gender varchar);
insert into gender (gender) select case when random<0.50 then 'female' when
random<0.99 then 'male' else 'other' end from (select random() as random,
generate_series(1,:table_size)) as subselect;
create index gender_btree on gender using btree (gender);
create index gender_hash on gender using hash (gender);
commit;
vacuum full analyze;
Vacuum full is not necessary here, just a little vodoo programming. I expected
that the hash index will be much smaller and quicker than the btree index,
because it doesn’t keep values inside itself, only hashes. But:
=> \d+
List of relations
Schema | Name | Type | Owner | Size | Description
--------+--------+-------+-------+-------+-------------
public | gender | table | olleg | 35 MB |
(1 row)
=> \di+
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+--------------+-------+-------+--------+-------+-------------
public | gender_btree | index | olleg | gender | 21 MB |
public | gender_hash | index | olleg | gender | 47 MB |
(2 rows)
The hash index not only is more than the btree index, but also is bigger than
the table itself. What is wrong with the hash index?
