On Fri, Jan 10, 2020 at 12:03:39PM +0100, Marco Colli wrote: > I have added this index which would allow an index only scan: > "index_subscriptions_on_project_id_and_created_at_and_tags" btree > (project_id, created_at DESC, tags) WHERE trashed_at IS NULL Are those the only columns in subscriptions ? > But Postgresql continues to use this index (which has less information and > then requires slow access to disk): > "index_subscriptions_on_project_id_and_created_at" btree (project_id, > created_at DESC) Did you vacuum the table ? Did you try to "explain" the query after dropping the 1st index (like: begin; DROP INDEX..; explain analyze..; rollback). Also, is the first (other) index btree_gin (you can \dx to show extensions) ? I think it needs to be a gin index to search tags ? On Fri, Jan 10, 2020 at 01:42:24PM +0100, Marco Colli wrote: > I would like to try your solution but I read that ALTER TABLE... SET > STATISTICS locks the table... Since it is just an experiment and we don't > know if it actually works it would be greate to avoid locking a large table > (50M) in production. I suggest to CREATE TABLE test_subscriptions (LIKE subscriptions INCLUDING ALL); INSERT INTO test_subscriptions SELECT * FROM subscriptions; ANALYZE test_subscriptions; Anyway, ALTER..SET STATS requires a strong lock but for only a brief moment (assuming it doesn't have to wait). Possibly you'd be ok doing SET statement_timeout='1s'; ALTER TABLE.... > Does CREATE STATISTICS lock the table too? You can check by SET client_min_messages=debug; SET lock_timeout=333; SET log_lock_waits=on; Looks like it needs ShareUpdateExclusiveLock. > Does statistics work on an array field like tags? (I can't find any > information) It think it'd be data type agnostic. And seems to work with arrays. On Fri, Jan 10, 2020 at 02:30:27PM +0100, Marco Colli wrote: > @Justin Pryzby I have tried this as you suggested: > > CREATE STATISTICS statistics_on_subscriptions_project_id_and_tags ON > project_id, tags FROM subscriptions; > VACUUM ANALYZE subscriptions; > > Unfortunately nothing changes and Postgresql continues to use the wrong > plan (maybe stats don't work well on array fields like tags??). It'd help to see SELECT stxddependencies FROM pg_statistic_ext WHERE stxoid='subscriptions'::regclass
