Обсуждение: Index not used. WHY?

On Thu, 4 Dec 2003, Andrei Bintintan wrote:

> Hi,
>
> I have the following table:
> CREATE TABLE public.rights (
> id int4 DEFAULT nextval('"rights_id_seq"'::text) NOT NULL,
> id_user int4 NOT NULL,
> id_modull int4 NOT NULL,
> CONSTRAINT rights_pkey PRIMARY KEY (id)
> )
>
> and I created the following indexes:
>
> CREATE INDEX right_id_modull_idx ON rights USING btree (id_modull);
> CREATE INDEX right_id_user_idx ON rights USING btree (id_user);
>
> Now the problem:
>
> EXPLAIN SELECT * FROM rights r WHERE r.id_modull =15
> returnes:
> Seq Scan on rights r (cost=0.00..12.30 rows=42 width=12)
> Filter: (id_modull = 15)
>
> EXPLAIN SELECT * FROM rights r WHERE r.id_user =15
> returnes:
> Index Scan using right_id_user_idx on rights r (cost=0.00..8.35 rows=11 width=12)
> Index Cond: (id_user = 15)
>
> Question: Why the right_id_modull_idx is NOT USED at the 1st query and
> the second query the right_id_user_idx index is used.

As a note, pgsql-performance is a better list for these questions.

So, standard questions:

How many rows are in the table, what does EXPLAIN ANALYZE show for the
queries, if you force index usage (set enable_seqscan=off) on the first
what does EXPLAIN ANALYZE show then, have you used ANALYZE/VACUUM ANALYZE
recently?