Re: Problems with non use of indexes

Hi,

On 2 Březen 2012, 13:12, Tyler Durden wrote:
> Hi,
> I can't figure out why query planner doesn't use the proper index, anyone
> can help me?
>
> This query properly uses indexes:
>
> mydb=# EXPLAIN SELECT U0."object_id" FROM "activity_follow" U0 WHERE
> (U0."content_type_id" = 3 AND U0."user_id" = 1);
>
> QUERY PLAN
> --------------------------------------------------------------------------------------------------------
> Index Scan using activity_follow_user_id on activity_follow u0
> (cost=0.00..4875.15 rows=4898 width=4)
> Index Cond: (user_id = 1)
> Filter: (content_type_id = 3)
> (3 rows)
>
> But the same query on a "IN" statement doesn't. The query planner uses Seq
> Scan on *U0."user_id" = 1*
>
> mydb=# EXPLAIN SELECT "activity_action"."id",
> "activity_action"."actor_id",
> "activity_action"."verb", "activity_action"."action_content_type_id",
> "activity_action"."action_object_id",
> "activity_action"."target_content_type_id",
> "activity_action"."target_object_id", "activity_action"."public",
> "activity_action"."created", "auth_user"."id", "auth_user"."username",
> "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email",
> "auth_user"."password", "auth_user"."is_staff", "auth_user"."is_active",
> "auth_user"."is_superuser", "auth_user"."last_login",
> "auth_user"."date_joined" FROM "activity_action" INNER JOIN "auth_user" ON
> ("activity_action"."actor_id" = "auth_user"."id") WHERE
> "activity_action"."actor_id" IN (SELECT U0."object_id" FROM
> "activity_follow" U0 WHERE (U0."content_type_id" = 3 AND *U0."user_id"
> = 1*)) ORDER BY "activity_action"."created" DESC LIMIT 100;
>
> QUERY PLAN
> --------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=9206.97..9207.22 rows=100 width=155)
> -> Sort (cost=9206.97..9320.34 rows=45347 width=155)
> Sort Key: activity_action.created
> -> Hash Join (cost=5447.39..7473.84 rows=45347 width=155)
> Hash Cond: (activity_action.actor_id = auth_user.id)
> -> Nested Loop (cost=4887.39..5020.58 rows=45347
> width=55)
> -> HashAggregate (cost=4887.39..4887.41 rows=2
> width=4)
> -> Index Scan using activity_follow_user_id on
> activity_follow u0 (cost=0.00..4875.15 rows=4898 width=4)
> Index Cond: (user_id = 1)
> Filter: (content_type_id = 3)
> -> Index Scan using activity_action_actor_id on
> activity_action (cost=0.00..65.20 rows=111 width=51)
> Index Cond: (activity_action.actor_id =
> u0.object_id)
> -> Hash (cost=278.00..278.00 rows=10000 width=104)
> -> Seq Scan on auth_user (cost=0.00..278.00
> rows=10000 width=104)
>
>
> If I do a SET enable_seqscan TO 'off'; It uses the index but is also slow.

Errr, what? The only sequential scan in that explain output is on
auth_user, not activity_follow which is the table referenced in the
original query. It actually uses index scan to read activity_follow

-> Index Scan using activity_follow_user_id on
activity_follow u0 (cost=0.00..4875.15 rows=4898
width=4)
Index Cond: (user_id = 1)
Filter: (content_type_id = 3)

kind regards
Tomas