[postgres@pgserver ~]$ psql
psql (10beta2)
Type "help" for help.

postgres=# select current_user,session_user;
 current_user | session_user 
--------------+--------------
 postgres     | postgres
(1 row)

postgres=# set role to u_3;
SET
postgres=> select current_user,session_user;
 current_user | session_user 
--------------+--------------
 u_3          | postgres
(1 row)

postgres=> 

注意： 
current_user 是 the current user identifier 
session_user 是 the current session user identifier  ，注：我理解下来，就是启动该session的user。

参考自：
https://www.postgresql.org/docs/10/static/sql-set-role.html