Really SLOW using GROUP BY ...!?

Hi,

I would like to know if someone have a solution for me ...

I have a table with today about 2,8 millions records.
The table have good indexes, in our case on idcond, and points !

When I do :
select sum(points) from gains;

With Oracle : 8 sec
With PostGreSQL : 10 sec

OK for this it's ok ...

But when I do :
select sum(points) from gains group by idcond;

With Oracle : 22 sec
With PostGreSQL : about 3 minutes !!!

I have done a vacuum analyse of the table gains ... just before testing
...
What can I do to optimize that result ?

Why the group by function is so slow ??

Thanks for your help !

Regards,
--
Hervé