To: "JS Ubei" <jsubei@yahoo.fr>
Cc: <pgsql-performance@postgresql.org>
Subject: Re: optimize query with a maximum(date) extraction
In-Reply-To: <87abs1qrhu.fsf@oxford.xeocode.com> (Gregory Stark's message of
	"Wed\, 05 Sep 2007 12\:30\:21 +0100")
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	<87abs1qrhu.fsf@oxford.xeocode.com>
From: Gregory Stark <stark@enterprisedb.com>
Organization: EnterpriseDB
Date: Wed, 05 Sep 2007 13:06:01 +0100
Message-ID: <87642pqpue.fsf@oxford.xeocode.com>
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"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