Re: Query planner wants to use seq scan

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