Optimize expresiions.

Hello,

Given a table:
create table atable (
  item integer;
);
and a view:
create view aview as select
  item,
  very_expensive_function(item) as exp,
  cheap_function(item) as cheap
from atable;

Now the query:
select item from aview where exp > 0 and cheap > 0;
will lead to a sequential scan on atable with filter:
very_expensive_function(item) > 0 and cheap_function(item) > 0

The query would run much faster with the filter reordered.
Is there a way to tell the planner/optimizer that certain functions are more
expensive than others, and should be postponed in lazy evaluation ?
Or is there a hook in the system that would allow me too look at and maybe
reorder expressions before they are executed ?

Cheers,

Han Holl