I am trying different solutions and what I have found is even more surprising to me...
The query is always this:
"index_subscriptions_on_project_id_and_created_at_and_tags" btree (project_id, created_at DESC, tags) WHERE trashed_at IS NULL
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)
On Fri, Jan 10, 2020 at 4:06 AM Justin Pryzby <pryzby@xxxxxxxxxxxxx> wrote:
On Fri, Jan 10, 2020 at 02:11:14AM +0100, Marco Colli wrote:
> I have a query on a large table that is very fast (0s):
> https://gist.github.com/collimarco/039412b4fe0dcf39955888f96eff29db#file-fast_query-txt
ORDER BY + LIMIT is a query which sometimes has issues, you can probably find
more by searching. The planner thinks it'll hit the LIMIT pretty soon and only
run a fraction of the index scan - but then it turns out to be wrong.
You might have poor statistics on project_id and/or tags. This *might* help:
ALTER TABLE subscriptions ALTER project_id SET STATISTICS 2000; ANALYZE subscriptions;
But I'm guessing there's correlation between the two, which the planner doesn't
know. If you're running at least v10, I'm guessing it would help to CREATE
STATISTICS on those columns (and analyze).
See one similar problem here (not involving LIMIT).
https://www.postgresql.org/message-id/flat/CABFxtPedz4zL%2BaPWut4%2B%3Dum4av1aAXr6OVRfRB_6K7mJKMbEcw%40mail.gmail.com
