Accessing tables from different databases

Hi all,
        I want to access a table-1 of database-1 from
        database-2. Is it possible in pgsql. If so,
        is it possible to define a foreign key constraint
        for a column in table-2 in database-2 which
        references to a column in table-1 in database-1.
TIA

regards,
Deepa K.