方法1：
select * from current_user;
eg：
kerrydb=> select * from current_user;
current_user
--------------
test
(1 row)
方法2：
select user;
eg：
kerrydb=> select user;
user
------
test
(1 row)
kerrydb=>
方法3：
\c
eg：
kerrydb=> \c
You are now connected to database "kerrydb" as user "test".
此方法只适用于psql命令。
\conninfo
eg：
postgres=# \conninfo
You are connected to database "postgres" as user "postgres" on host "127.0.0.1" at port "5432".
方法4：
current_user是the current user identifier
session_user是the current session user identifier。
postgres=# SELECT session_user, current_user;
session_user | current_user
--------------+--------------
postgres | postgres
(1 row)