dblink EXTENSION is a technique to connect another database and make operation of this database so to do that you need:

1-Create a dblink extention:

CREATE EXTENSION dblink;

2-Make your operation:

For exemple Select some attribute from another table in another database:

SELECT * FROM 
dblink ('dbname = bd_distance port = 5432 host = 10.6.6.6 user = username 
password = passw@rd', 'SELECT id, code FROM schema.table') 
AS newTable(id INTEGER, code character varying);