[BUGS] BUG #14469: Wrong cost estimates for merge append plan withpartitions.
От | maxim.boguk@gmail.com |
---|---|
Тема | [BUGS] BUG #14469: Wrong cost estimates for merge append plan withpartitions. |
Дата | |
Msg-id | 20161217105346.13336.97087@wrigleys.postgresql.org обсуждение исходный текст |
Ответы |
Re: [BUGS] BUG #14469: Wrong cost estimates for merge append planwith partitions.
|
Список | pgsql-bugs |
The following bug has been logged on the website: Bug reference: 14469 Logged by: Maksym Boguk Email address: maxim.boguk@gmail.com PostgreSQL version: 9.6.1 Operating system: Linux Description: The problem query (which never finishes because of huge tables involved): explain SELECT "feed_actions".* FROM "feed_actions" WHERE (requested_at > '2016-12-17 09:44:17.042659') ORDER BY "feed_actions"."created_at" DESC LIMIT 1; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------ Limit (cost=2.47..853.03 rows=1 width=697) -> Merge Append (cost=2.47..10603118.78 rows=12466 width=697) Sort Key: feed_actions.created_at DESC -> Index Scan Backward using feed_actions_created_at_idx on feed_actions (cost=0.12..8.14 rows=1 width=1184) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan Backward using feed_actions_2016_09_created_at_idx on feed_actions_2016_09 (cost=0.56..3212964.64 rows=1 width=202) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan Backward using feed_actions_2016_10_created_at_idx on feed_actions_2016_10 (cost=0.56..3612797.62 rows=1 width=202) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan Backward using feed_actions_2016_11_created_at_idx on feed_actions_2016_11 (cost=0.56..2998680.21 rows=1 width=214) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan Backward using feed_actions_2016_12_created_at_idx on feed_actions_2016_12 (cost=0.43..778269.49 rows=12442 width=696) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan Backward using feed_actions_2017_01_created_at_idx on feed_actions_2017_01 (cost=0.14..45.19 rows=20 width=1184) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) In the same time (for all partitions except feed_actions_2016_12) there are no rows with requested_at > '2016-12-17 09:44:17.042659': explain analyze select * from feed_actions_2016_09 "feed_actions" WHERE (requested_at > '2016-12-17 09:44:17.042659'); QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Index Scan using feed_actions_2016_09_requested_at_idx on feed_actions_2016_09 feed_actions (cost=0.56..4.60 rows=1 width=202) (actual time=0.006..0.006 rows=0 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) Planning time: 0.094 ms Execution time: 0.024 ms What's more, selecting from the single partition with the same order by/limit - producing expected/fast plan: explain analyze select * from feed_actions_2016_09 "feed_actions" WHERE (requested_at > '2016-12-17 09:44:17.042659') ORDER BY "feed_actions"."created_at" DESC LIMIT 1;; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Limit (cost=4.61..4.61 rows=1 width=202) (actual time=0.012..0.012 rows=0 loops=1) -> Sort (cost=4.61..4.61 rows=1 width=202) (actual time=0.011..0.011 rows=0 loops=1) Sort Key: created_at DESC Sort Method: quicksort Memory: 25kB -> Index Scan using feed_actions_2016_09_requested_at_idx on feed_actions_2016_09 feed_actions (cost=0.56..4.60 rows=1 width=202) (actual time=0.006..0.006 rows=0 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) Planning time: 0.126 ms Execution time: 0.052 ms So statistics - are correct, but somehow the database think that search for the 0 (may by 1) row via scanning whole partition with backward index scan - will be fast. Question - what's wrong with merge append cost estimates in that case? Play with planner cost* setting - have no effect on the plan selection. PS: good/fast plan could be produced via common hack: explain analyze SELECT "feed_actions".* FROM "feed_actions" WHERE (requested_at > '2016-12-17 09:44:17.042659') ORDER BY "feed_actions"."created_at"+'0 second'::interval DESC LIMIT 1; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Limit (cost=1212.68..1212.69 rows=1 width=705) (actual time=32.134..32.134 rows=1 loops=1) -> Sort (cost=1212.68..1243.86 rows=12470 width=705) (actual time=32.133..32.133 rows=1 loops=1) Sort Key: ((feed_actions.created_at + '00:00:00'::interval)) DESC Sort Method: top-N heapsort Memory: 25kB -> Result (cost=0.00..1150.33 rows=12470 width=705) (actual time=0.052..24.068 rows=8397 loops=1) -> Append (cost=0.00..994.46 rows=12470 width=697) (actual time=0.047..16.364 rows=8397 loops=1) -> Seq Scan on feed_actions (cost=0.00..0.00 rows=1 width=1184) (actual time=0.003..0.003 rows=0 loops=1) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan using feed_actions_2016_09_requested_at_idx on feed_actions_2016_09 (cost=0.56..4.60 rows=1 width=202) (actual time=0.007..0.007 rows=0 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan using feed_actions_2016_10_requested_at_idx on feed_actions_2016_10 (cost=0.56..4.60 rows=1 width=202) (actual time=0.004..0.004 rows=0 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan using feed_actions_2016_11_requested_at_idx on feed_actions_2016_11 (cost=0.56..6.85 rows=1 width=214) (actual time=0.004..0.004 rows=0 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Index Scan using feed_actions_2016_12_requested_at_idx on feed_actions_2016_12 (cost=0.43..967.66 rows=12446 width=696) (actual time=0.027..13.166 rows=8397 loops=1) Index Cond: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) -> Seq Scan on feed_actions_2017_01 (cost=0.00..10.75 rows=20 width=1184) (actual time=0.005..0.005 rows=0 loops=1) Filter: (requested_at > '2016-12-17 09:44:17.042659'::timestamp without time zone) Planning time: 1.020 ms Execution time: 32.370 ms -- Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-bugs
В списке pgsql-bugs по дате отправления:
Предыдущее
От: Mark KirkwoodДата:
Сообщение: Re: [BUGS] pg_dump's results have quite different size
Следующее
От: Alexey ErmakovДата:
Сообщение: Re: [BUGS] BUG #14469: Wrong cost estimates for merge append planwith partitions.