Re: Query planner wants to use seq scan

[Alex Ignatov <a.ignatov@xxxxxxxxxxxxxx>]

On 27.10.2015 14:10, Bertrand Paquet wrote:

Yes, I have run VACUUM ANALYZE, no effect.

Bertrand

2015-10-27 12:08 GMT+01:00 Alex Ignatov <a.ignatov@xxxxxxxxxxxxxx>:

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

What is the result of
select relname,n_live_tup,n_dead_tup, last_vacuum, last_autovacuum, last_analyze, last_autoanalyze from pg_stat_user_tables where relname='external_sync_messages' ?

-- 
Alex Ignatov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company