Example using dblink to execute a SELECT query between different databases. (PG 9.4)

postgres-dblink.sql

-- enable dblink support
CREATE EXTENSION dblink;

-- create the connection
SELECT dblink_connect('myconn', 'dbname=tracking-service');

-- then use the connection
SELECT "oauthId"
FROM "56_profile" local
INNER JOIN  dblink('myconn','SELECT DISTINCT user_id FROM "event"')
AS remote(user_id int)
ON local.id = remote.user_id;

Thanks for your example!
It was very helpful for me