Accessing other Databases

Hi all.
Anybody knows some way to access other database inside of a different
database(under the same instalation of postgres)?

Like the example under:
I have two database under the same instalation(database A and B). I am
doing some queries in A - pslq A - and inside A, I want to access, tables,
functions in B, is't possible? ( I know how to do this in Sybase)


Thanks

Osmar