Re: Left Join with Limit 1

From: Paul Jungwirth <pj(at)illuminatedcomputing(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: Re: Left Join with Limit 1
Date: 2015-10-12 15:12:58
Message-ID: 561BCDFA.90308@illuminatedcomputing.com
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> Running the queries individually and using a limit on the golite ip db
> results are back immediately 1-2ms but when using the first query it
> takes 2-3 seconds.
>
> Is there a way to use a limit in the join?

This sounds like the real issue is a missing/incorrect index, but if
you're on 9.4+ you can use a lateral join like this:

SELECT S.referrer_ip,
I.geoname_id
FROM viewing_stats AS S
LEFT JOIN LATERAL (
SELECT *
FROM geolite_city_ip4
WHERE S.referrer_ip::inet <<= network
LIMIT 1
) I
ON true
WHERE viewing_id=74;

You might also want some kind of ordering in that subquery so that the
results are deterministic.

Paul

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