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
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