Always knowing which user is logged in

 In the preceding recipes, we just logged the value of the user variable in the current PostgreSQL session to log the current user role.

This does not always mean that this particular user was the user that was actually authenticated at the start of the session. For example, a superuser can execute the SET  ROLE  TO ... command to set its current role to any other user or role in the system. As you might expect, non-superusers can assume only those roles that they own.

It is possible to differentiate between the logged-in role and the assumed role using the current_user and session_user session variables:

postgres=# select current_user, session_user;
current_user | session_user
-------------+--------------
postgres     | postgres 

postgres=# set role to bob;
SET
postgres=> select current_user, session_user;
current_user | session_user
-------------+--------------
bob          | postgres

Sometimes, it is desirable to let each user log in with their own username and just assume the role needed on a case-by-case basis.