From: Gregory Stark <stark@enterprisedb.com>
To: JS Ubei <jsubei@yahoo.fr>
Cc: pgsql-performance@postgresql.org
Subject: Re: optimize query with a maximum(date) extraction
Date: Wed, 05 Sep 2007 13:06:01 +0100
Message-ID: <87642pqpue.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <87abs1qrhu.fsf@oxford.xeocode.com>
References: <302443.65402.qm@web26209.mail.ukl.yahoo.com>
<87abs1qrhu.fsf@oxford.xeocode.com>
"Gregory Stark" <stark@enterprisedb.com> writes:
> "JS Ubei" <jsubei@yahoo.fr> writes:
>
>> I need to improve a query like :
>>
>> SELECT id, min(the_date), max(the_date) FROM my_table GROUP BY id;
>...
> I don't think you'll find anything much faster for this particular query. You
> could profile running these two (non-standard) queries:
>
> SELECT DISTINCT ON (id) id, the_date AS min_date FROM my_table ORDER BY id, the_date ASC
> SELECT DISTINCT ON (id) id, the_date AS max_date FROM my_table ORDER BY id, the_date DESC
Something else you might try:
select id,
(select min(the_date) from my_table where id=x.id) as min_date,
(select max(the_date) from my_table where id=x.id) as max_date
from (select distinct id from my_table)
Recent versions of Postgres do know how to use the index for a simple
ungrouped min() or max() like these subqueries.
This would be even better if you have a better source for the list of distinct
ids you're interested in than my_table. If you have a source that just has one
record for each id then you won't need an extra step to eliminate duplicates.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-performance@postgresql.org
Cc: stark@enterprisedb.com, jsubei@yahoo.fr
Subject: Re: optimize query with a maximum(date) extraction
In-Reply-To: <87642pqpue.fsf@oxford.xeocode.com>
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