implementing another hash join...

Hi,
Im new to postgres and I'm about to implement the algorithm "double 
pipelined hash join". I've started using SPI functions but soon I 
realised that  I 'm  not at the right way (am I?). Is there a way that 
I'm not obliged to change code of  the optimizer of postgres but to 
write a seperate code as a new function?I've been told by my supervisor 
that I should use the function that is doing the hashing of a relation.  
There is a chaos !!!!Please help!!I'm reading the documentation of 
postgres and some '.c .h'  files (mainly the createplan.c  at the 
directory "~/src/backend/optimizer/plan/") to figer out  what to do and 
from where to start.
Regards!!!!
Martha