bitset counting as a user defined function in postgresql 9.2?

As an initial attempt to try hands-on with postgresql, I've learned on user
created functions and some other internals of postgresql.

Few materials on postgresql indicate that the aggregate functions are slow
due to some internal reasons (index covering, null etc).

I'm looking forward to implement my own bit counting or counting mechanism
(I've gone through some bit twiddling as well) via user defined functions
(in C) and in cases I may need to create user defined data types as well.

Are there any possible scenarios where user defined count would perform
better than the underlying implementation? Is it worth a try?

And are there any better ways to evaluate, compare both? (other than
EXPLAIN ANALYZIS)

Please guide.

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Regards,
K.Kanarupan
Undergraduate,
Dept. of Computer Science & Engineering
University of Moratuwa

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