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Re: Slow query with backwards index scan

From: Jeremy Harris <jgh(at)wizmail(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow query with backwards index scan
Date: 2007-07-28 19:40:12
Message-ID: 46AB9B9C.8070009@wizmail.org (view raw or flat)
Thread:
Lists: pgsql-performance
Tilmann Singer wrote:
> * andrew(at)pillette(dot)com <andrew(at)pillette(dot)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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