From: Tilmann Singer <tils-pgsql@tils.net>
To: andrew@pillette.com
Cc: pgsql-performance@postgresql.org
Subject: Re: Slow query with backwards index scan
Date: Sat, 28 Jul 2007 21:27:13 +0200
Message-ID: <20070728192713.GH19392@tils.net> (raw)
In-Reply-To: <200707281903.l6SJ3qs07818@pillette.com>
References: <200707281903.l6SJ3qs07818@pillette.com>
* andrew@pillette.com <andrew@pillette.com> [20070728 21:05]:
> Let's try putting the sort/limit in each piece of the UNION to speed them up separately.
>
> SELECT * FROM (
> (SELECT * FROM large_table lt
> WHERE lt.user_id = 12345
> ORDER BY created_at DESC LIMIT 10) AS q1
> UNION
> (SELECT * FROM large_table lt
> WHERE user_id IN (SELECT contact_id FROM relationships WHERE user_id=12345)
> ORDER BY created_at DESC LIMIT 10) AS q2
> ORDER BY created_at DESC LIMIT 10;
It's not possible to use ORDER BY or LIMIT within unioned queries.
http://www.postgresql.org/docs/8.2/static/sql-select.html#SQL-UNION
Would that make sense at all given the way the postgresql planner
works? Does that work in other DB's?
Til
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To: pgsql-performance@postgresql.org
Cc: tils-pgsql@tils.net, andrew@pillette.com
Subject: Re: Slow query with backwards index scan
In-Reply-To: <20070728192713.GH19392@tils.net>
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