Re: optimize query with a maximum(date) extraction

"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