Eu coloquei esse código no login.sql, que roda cada vez que abrimos um Sql*plus:
set termout off
col user_id new_value user_id
col user_name new_value user_name
select user_id, username user_name
from user_users
where username = user;
col db_name new_value db_name
select
lower(substr(global_name,1,(instr(global_name||'.','.',1,1)-1)))||':' db_name
from global_name;
set termout on
set sqlprompt "&db_name&user_name> "
SQL>
SQL> @login
dbglufke:SCOTT>
dbglufke:SCOTT>
dbglufke:SCOTT>