How to resolve db_link?

Question

PostgreSQL

On the test everything was created from the root, there were no problems. I don't understand how to set up data retrieval via db_link.
If you just make a request - SELECT * FROM dblink('dbname=tbl', '...');then

18:20:34 Kernel error: ERROR: password is required
DETAIL: Non-superuser cannot connect if the server does not request a password.

If add

SELECT * FROM dblink('dbname=db user=user password=pwd', '...')

- same error, strange.
If a

GRANT EXECUTE ON FUNCTION dblink_connect_u(TEXT, TEXT) TO user;

, and then

SELECT dblink_connect_u('c', 'dbname=db');
SELECT * FROM dblink('c', '...')

- the first time is fine. And then

18:24:03 Kernel error: ERROR: duplicate connection name

.
So what's right?