Re: Problems with non use of indexes

Hi,

On Fri, Mar 2, 2012 at 12:23 PM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:

> 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
>
>
Yes, but if I remove *U0."user_id" = 1 *will use the index:

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 )) ORDER BY
"activity_action"."created" DESC LIMIT 100;
QUERY
PLAN
----------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..4502.18 rows=100 width=155)
-> Nested Loop (cost=0.00..2041605.23 rows=45347 width=155)
-> Nested Loop Semi Join (cost=0.00..1907985.65 rows=45347
width=55)
-> Index Scan using activity_action_created on
activity_action (cost=0.00..40093.37 rows=1104800 width=51)
-> Index Scan using activity_follow_object_id on
activity_follow u0 (cost=0.00..5519.13 rows=3328 width=4)
Index Cond: (u0.object_id = activity_action.actor_id)
Filter: (u0.content_type_id = 3)
-> Index Scan using auth_user_pkey on auth_user (cost=0.00..2.93
rows=1 width=104)
Index Cond: (auth_user.id = activity_action.actor_id)