Unnecessary function calls

Hi,

when using LIMIT, how do I tell the planner to only call a function for
rows it returns?

An example: I want to fetch the top five categories. A function
get_category_text_path(cat_id int) returns the textual representation of
the category. For that I do something like:

SELECT id, get_category_text_path(id)
   FROM category
   ORDER BY rank
   LIMIT 5

Unfortunately this takes very long because it calls
get_category_text_path() for all of the 450'000 categories in the table.
But I only need the full text path of the top five rows.

It does not matter if I declare the function to be IMMUTABLE, STABLE or
VOLATILE - it gets called for every row in category (which normally is
what you want I guess).

How can I rewrite the query to call get_category_text_path() only for
the top five rows?

Thanks for hints.

Markus