Re: optimize query with a maximum(date) extraction - Mailing list pgsql-performance

"JS Ubei" <jsubei@yahoo.fr> writes:

> Hi all,
>
> I need to improve a query like :
>
> SELECT id, min(the_date), max(the_date) FROM my_table GROUP BY id;
>
> Stupidly, I create a B-tree index on my_table(the_date), witch is logically
> not used in my query, because it's not with a constant ? isn't it ?

That's not so stupid, it would be possible for a database to make use of such
an index for this query. But it's not one of the plans Postgres knows how to
execute.

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

I think the first of these can actually use your index but the latter can't
unless you create one for it specifically (which is not so easy -- it'll be
easier in 8.3 though). Worse, I'm not really sure it'll be any faster than the
query you already have.

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com