Request with min aggregate in Postgres

[Gwenaelle]

It's me again with my Postgres 7.4 database and my quest to speed up graph and screen display for long periods.
I saw requests like this many times :

select min(clock) from history where itemid=18356

and the same from table trends or history_uint, etc.

This request takes a bit too much time (more than a second).
I read Postgres 7.4 documentation and it says that Postgres don't use indexes for aggregates functions because users can create there own aggregates.

http://www.postgresql.org/docs/7.4/interactive/functions-aggregate.html

It says that to have best results it is recommended to replace a request like :

SELECT min(col) FROM sometable;

with :

SELECT col FROM sometable ORDER BY col ASC LIMIT 1;

I don't know if it's the same for latest 8.3 version, documentation is not as precise on this point.

http://www.postgresql.org/docs/8.3/interactive/functions-aggregate.html

Thanks for your lights.