UDF in C slow

Hi all,

I have implemented a user-defined function in C that returns a boolean
value after some computation. Now I have a query Q such that when I
specify the function in the WHERE clause of Q, Q runs in 40 secs and if
I don't use the function it runs in 4 secs. Then I thought that my
implementation of this function could be slow; so I decided to write a
very simple function that just returns true without any computation. To
my surprise, it also takes around 40 seconds to run Q with the new very
simple function. Does anybody have a clue about what might be going wrong?

Cheers,

Inanc