Slow response on a query

[Quoted] I´ve a query which looks like this

select a
from b
where c = value

I also have an index for that query, and this works really fast.

But if I replace my query:

select a
from b
where c is not null

it becomes really slow!!!. So my question is: Is there a way so I create an index for that query (over nulls fileds in a table)

Thanks a lot

-- 
Atte.
Álvaro Palma Aste
Grupo de Ing. Biomédica
Depto. de Ing. Eléctrica - U. de Chile
apalma_at_cec.uchile.adivina