On 16/01/15 11:30, Josh Berkus wrote:
This is an obfuscation and mock up, but:table files ( id serial pk, filename text not null, state varchar(20) not null ... 18 more columns ) index file_state on (state) (35GB in size) index file_in_flight_state (state) where state in ( 'waiting','assigning', 'processing' ) (600MB in size) ... 10 more indexes More important facts: * state = 'done' 95% of the time. thereform the partial index represents only 5% of the table * all indexes and the table are very bloated * server has 128GB RAM * Version 9.2. Given this setup, I would expect the planner to *always* choose file_in_flight_state over file_state for this query: SELECT id, filename FROM files WHERE state = 'waiting'; ... and yet it keeps selecting file_state based on extremely small changes to the stats. This is important because the same query, using file_state, is 20X to 50X slower, because that index frequently gets pushed out of memory. What am I missing? Or is this potentially a planner bug for costing?
Are you seeing a bitmapscan access plan? If so see if disabling it gets you a plan on the files_in_flight index. I'm seeing this scenario with a fake/generated dataset a bit like yours in 9.2 (9.5 uses the files_in_flight w/o any coercing).
regards Mark -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
