Question

Is it possible to JOIN rows from two separate postgres databases?

I am working with system with couple databases in one server and sometimes I really need such a feature.

Answer 1

According to http://wiki.postgresql.org/wiki/FAQ

There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.
contrib/dblink allows cross-database queries using function calls. Of
course, a client can also make simultaneous connections to different
databases and merge the results on the client side.

EDIT: 3 years later (march 2014), this FAQ entry has been revised and is more helpful:

How do I perform queries using multiple databases?

There is no way to directly query a database other than the current
one. Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.

The SQL/MED support in PostgreSQL allows a “foreign data wrapper” to
be created, linking tables in a remote database to the local database.
The remote database might be another database on the same PostgreSQL
instance, or a database half way around the world, it doesn’t matter.
postgres_fdw is built-in to PostgreSQL 9.3 and includes read/write
support; a read-only version for 9.2 can be compiled and installed as
a contrib module.

contrib/dblink allows cross-database queries using function calls and
is available for much older PostgreSQL versions. Unlike postgres_fdw
it can’t “push down” conditions to the remote server, so it’ll often
land up fetching a lot more data than you need.

Of course, a client can also make simultaneous connections to
different databases and merge the results on the client side.