From: Ezequiel Tolnay <mail@etolnay.com.ar>
To: pgsql-sql@postgresql.org
Subject: Re: Trimming the cost of ORDER BY in a simple query
Date: Tue, 03 May 2005 18:08:23 +1000
Message-ID: <d57bhc$2nfr$1@news.hub.org> (raw)
In-Reply-To: <1115104938.5853.25.camel@localhost.localdomain>
References: <20050503043531.23516.qmail@web31106.mail.mud.yahoo.com>
<1115104938.5853.25.camel@localhost.localdomain>
Ragnar Hafstað wrote:
> another possibility (total guess) is a functional index
> create index func_id_date on user_url(user_id,(add_date::date));
> SELECT ... ORDER BY uu.user_id DESC ,uu.add_date::date DESC LIMIT 7;
> I have no idea if this will work, or if the planner will use such
> an index.
To make sure the index is used you could build the query using a
subquery like this:
SELECT DISTINCT date_part('year', add_date), date_part('month',
add_date), date_part('day', add_date)
FROM (
SELECT uu.add_date::date as add_date
FROM user_url uu
GROUP BY uu.user_id, uu.add_date
HAVING uu.user_id=1
ORDER BY uu.add_date::date DESC
LIMIT 7
) x;
Perhaps a select distinct instead of the group by in the subquery would
make use the index?
Cheers,
Ezequiel Tolnay
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Subject: Re: Trimming the cost of ORDER BY in a simple query
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