postgres optimizer

Hi:
I am working on postgres optimizer code to add some statistics
collection features for an academic project. At some point in my work, I
need to be able to tell if a join is a foreign key join or not. Also in
case of a foreign key join, I need to identify which input to the join
operator has the primary key attribute and which one has teh foreign
key. In other words, when we are making a JoinPath struct, I need to
identify if the join between the inner and outer RelOptInfo structs is a
foreign key join and if so, which RelOptInfo has the primary key. Does
anyone know the best way to do this? Should I go to "joinrestrictinfo"
field and go over all expressions? Is there any easier way?

Regards,
HSA