Re: Indexes with condition using immutable functions applied to column not used - Mailing list pgsql-performance

On Tue, 2011-02-08 at 06:15 +0100, Jesper Krogh wrote:
> On 2011-02-08 01:14, Sylvain Rabot wrote:
> > CREATE INDEX directory_id_user_mod_cons_hash_0_btree_idx ON mike.directory USING btree (id_user) WHERE
__mod_cons_hash(id_user,4) = 0; 
> > CREATE INDEX directory_id_user_mod_cons_hash_1_btree_idx ON mike.directory USING btree (id_user) WHERE
__mod_cons_hash(id_user,4) = 1; 
> > CREATE INDEX directory_id_user_mod_cons_hash_2_btree_idx ON mike.directory USING btree (id_user) WHERE
__mod_cons_hash(id_user,4) = 2; 
> > CREATE INDEX directory_id_user_mod_cons_hash_3_btree_idx ON mike.directory USING btree (id_user) WHERE
__mod_cons_hash(id_user,4) = 3; 
> >
>
> > mike=# EXPLAIN ANALYZE SELECT * FROM directory WHERE id_user = 4;
>
> Should be written as:
> select * from directory where __mod_cons_hash(id_user,4) = 4%4;
>
> Then it  should just work.
>
> --
> Jesper
>

The where clause you wrote selects all the directory records that have a
id_user % 4 equivalent to 0 like 0, 4, 8, 16 ... etc. It does use the
indexes but it is not was I want to select.

--
Sylvain Rabot <sylvain@abstraction.fr>