order by and aggregate

Hi
I created my own aggregate function working as max(sum(value))
It adds positive and negative values and finds maximum of this sum.
To work properly this function needs data to be sorted.

select  maxsum(value)
from some_table  order by some_field

doesn't work:
ERROR:  Attribute some_table.some_field must be GROUPed or used in an 
aggregate function


I found I can obey this using subselect:
select  maxsum(X.value)
from (select value  from some_table  order by some_field) X

I can't create subselect, because I want to change this query into a 
view. In my case postgresql doesn't use indexes on subselect inside a view.

What should I do?

Regards,
Tomasz Myrta