Compiling Functions in Postgresql

Hi,

I know that in Sql Server, Stored Procedures are compiled only once and then the same execution plan is utilized. This definitely gives a tremendous performance boost
when Stored Procedures are called repeatedly.

Now, in Postgresql, how are the User Defined Functions compiled. Is it just once or every single time a function is called.
I used Function instead of Stored Procedure while talking about Postgresql because Stored Procedures in Postgresql are Wrapped around in
a User Defined Function.

Please let me know.

Thanks,
Jas