I've got an error message while dropping the user: "ORA-01940: cannot drop a user that is currently connected." FYI, I tried to drop user ALPHAPOS2. It is the master user for my server. What will come next if I run this command:
ALTER USER ALPHAPOS2 ACCOUNT LOCK
Is there any problem that would arise to my server performance?
Locking a user account will not affect your server performance. It will only stop the user from reconnecting to your database again. This will not help you get around the ORA-1940 error as locking the account will not affect any currently connected sessions. You will have to kill the current sessions for this user. But before you do that, make sure that the user is not needed. I would be very suspicious of dropping a user that someone is currently using.