Cross database queries

Hi

Suppose I have two databases on one server (lets call them DB_A and DB_B)
and I was to write a cross database query. How do I do this in PostgreSQL?

On MS-SQL I would do something like:

SELECT ta.Field1, ta.Field2, tb.Field2
FROM DB_A.dbo.SomeTable ta
JOIN DB_B.dbo.SomeOtherTable tb
  ON ta.Field1 = tb.Field1

Thanks
Craig