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