From: Jeremy Harris <jgh@wizmail.org>
To: pgsql-performance@postgresql.org
Subject: Re: Slow query with backwards index scan
Date: Sat, 28 Jul 2007 20:40:12 +0100
Message-ID: <46AB9B9C.8070009@wizmail.org> (raw)
In-Reply-To: <20070728192713.GH19392@tils.net>
References: <200707281903.l6SJ3qs07818@pillette.com>
<20070728192713.GH19392@tils.net>
Tilmann Singer wrote:
> * 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
"ORDER BY and LIMIT can be attached to a subexpression if it is enclosed in parentheses"
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To: pgsql-performance@postgresql.org
Cc: jgh@wizmail.org
Subject: Re: Slow query with backwards index scan
In-Reply-To: <46AB9B9C.8070009@wizmail.org>
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