index on function

Hi,

I'm doing a course project extending PostgreSQL
somehow. One part of this is using an index on a
function. I thought it's easy to create an function
index initially. But later found that PostgreSQL only
allow functions taking one or more columns. But the
function I need to create index on takes composite
type. Specifically, a relation R, a user-defined
function f, f takes R (the composite type as its
argument), f returns a float. f is defined in C. I
want to create a function index like this:

create index idx on R (f(R))

But it doesn't pass PostgreSQL's grammar.

Any suggestion?

__________________________________
Do you Yahoo!?
The New Yahoo! Search - Faster. Easier. Bingo.
http://search.yahoo.com