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Re: Bad query plan when you add many OR conditions - Mailing list pgsql-performance

From Marco Colli
Subject Re: Bad query plan when you add many OR conditions
Date
Msg-id CAFvCgN55e_USxidsShLX7_Eu-t8n31LL6LdsodxvX1WpHuLbag@mail.gmail.com
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In response to Re: Bad query plan when you add many OR conditions  (Justin Pryzby <pryzby@telsasoft.com>)
Responses Re: Bad query plan when you add many OR conditions  (Tomas Vondra <tomas.vondra@2ndquadrant.com>)
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@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??).

On Fri, Jan 10, 2020 at 4:06 AM Justin Pryzby <pryzby@telsasoft.com> 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

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