Re: Query planner wants to use seq scan

Yes, I have run VACUUM ANALYZE, no effect.

Bertrand

2015-10-27 12:08 GMT+01:00 Alex Ignatov <a(dot)ignatov(at)postgrespro(dot)ru>:

> On 27.10.2015 12:35, Bertrand Paquet wrote:
>
>> Hi all,
>>
>> We have a slow query. After analyzing, the planner decision seems to be
>> discutable : the query is faster when disabling seqscan. See below the two
>> query plan, and an extract from pg_stats.
>>
>> Any idea about what to change to help the planner ?
>>
>> An information which can be useful : the number on distinct value on
>> organization_id is very very low, may be the planner does not known that,
>> and take the wrong decision.
>>
>> Regards,
>>
>> Bertrand
>>
>> # explain analyze SELECT 1 AS one FROM "external_sync_messages" WHERE
>> "external_sync_messages"."organization_id" = 1612 AND
>> ("external_sync_messages"."status" NOT IN ('sent_to_proxy', 'in_progress',
>> 'ok')) AND "external_sync_messages"."handled_by" IS NULL LIMIT 1;
>>
>> QUERY PLAN
>>
>>
>> --------------------------------------------------------------------------------------------------------------------------------------------
>>
>> Limit (cost=0.00..12.39 rows=1 width=0) (actual time=232.212..232.213
>> rows=1 loops=1)
>>
>> -> Seq Scan on external_sync_messages (cost=0.00..79104.69 rows=6385
>> width=0) (actual time=232.209..232.209 rows=1 loops=1)
>>
>> Filter: ((handled_by IS NULL) AND (organization_id = 1612) AND
>> ((status)::text <> ALL ('{sent_to_proxy,in_progress,ok}'::text[])))
>>
>> Rows Removed by Filter: 600140
>>
>> Planning time: 0.490 ms
>>
>> Execution time: 232.246 ms
>>
>> (6 rows)
>>
>> # set enable_seqscan = off;
>>
>> SET
>>
>> # explain analyze SELECT 1 AS one FROM "external_sync_messages" WHERE
>> "external_sync_messages"."organization_id" = 1612 AND
>> ("external_sync_messages"."status" NOT IN ('sent_to_proxy', 'in_progress',
>> 'ok')) AND "external_sync_messages"."handled_by" IS NULL LIMIT 1;
>>
>> QUERY PLAN
>>
>>
>> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>>
>> Limit (cost=0.42..39.88 rows=1 width=0) (actual time=0.030..0.030
>> rows=1 loops=1)
>>
>> -> Index Scan using index_external_sync_messages_on_organization_id
>> on external_sync_messages (cost=0.42..251934.05 rows=6385 width=0) (actual
>> time=0.028..0.028 rows=1 loops=1)
>>
>> Index Cond: (organization_id = 1612)
>>
>> Filter: ((handled_by IS NULL) AND ((status)::text <> ALL
>> ('{sent_to_proxy,in_progress,ok}'::text[])))
>>
>> Planning time: 0.103 ms
>>
>> Execution time: 0.052 ms
>>
>> (6 rows)
>>
>> # SELECT attname, inherited, n_distinct,
>> array_to_string(most_common_vals, E'\n') as most_common_vals FROM pg_stats
>> WHERE tablename = 'external_sync_messages' and attname IN ('status',
>> 'organization_id', 'handled_by');
>>
>> attname | inherited | n_distinct | most_common_vals
>>
>> -----------------+-----------+------------+------------------
>>
>> handled_by | f | 3 | 3 +
>>
>> | | | 236140 +
>>
>> | | | 54413
>>
>> organization_id | f | 22 | 1612 +
>>
>> | | | 287 +
>>
>> | | | 967 +
>>
>> | | | 1223 +
>>
>> | | | 1123 +
>>
>> | | | 1930 +
>>
>> | | | 841 +
>>
>> | | | 1814 +
>>
>> | | | 711 +
>>
>> | | | 1513 +
>>
>> | | | 1794 +
>>
>> | | | 1246 +
>>
>> | | | 1673 +
>>
>> | | | 1552 +
>>
>> | | | 1747 +
>>
>> | | | 2611 +
>>
>> | | | 2217 +
>>
>> | | | 2448 +
>>
>> | | | 2133 +
>>
>> | | | 1861 +
>>
>> | | | 2616 +
>>
>> | | | 2796
>>
>> status | f | 6 | ok +
>>
>> | | | ignored +
>>
>> | | | channel_error +
>>
>> | | | in_progress +
>>
>> | | | error +
>>
>> | | | sent_to_proxy
>>
>> (3 rows)
>>
>> # select count(*) from external_sync_messages;
>>
>> count
>>
>> --------
>>
>> 992912
>>
>> (1 row)
>>
>>
>> Hello, Bertrand!
> May be statistics on external_sync_messages is wrong? i.e planner give us
> rows=6385 but seq scan give us Rows Removed by Filter: 600140
> Maybe you should recalc it by VACUUM ANALYZE it?
>
> --
> Alex Ignatov
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>