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Re: Slow query with backwards index scan

From: Tilmann Singer <tils-pgsql(at)tils(dot)net>
To: andrew(at)pillette(dot)com
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow query with backwards index scan
Date: 2007-07-28 19:27:13
Message-ID: 20070728192713.GH19392@tils.net (view raw or flat)
Thread:
Lists: pgsql-performance
* 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

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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Subject: select on 1milion register = 6s
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