Message-ID: <46ABA1AA.7020701@emolecules.com>
Date: Sat, 28 Jul 2007 13:06:02 -0700
From: Craig James <craig_james@emolecules.com>
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To: Tilmann Singer <tils-pgsql@tils.net>
CC: andrew@pillette.com,  pgsql-performance@postgresql.org
Subject: Re: Slow query with backwards index scan
References: <200707281903.l6SJ3qs07818@pillette.com>
	<20070728192713.GH19392@tils.net>
In-Reply-To: <20070728192713.GH19392@tils.net>
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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

If I'm reading this documentation correctly, it *is* possible, as long as they're inside of a sub-select, as in this case.

Craig