something interesting when read docs

When you kill a session with "ALTER SYSTEM KILL SESSION  '' ".

If the session is performing some activity that must be completed, such as waiting for
a reply from a remote database or rolling back a transaction, then Oracle Database
waits for this activity to complete, marks the session as terminated, and then returns
control to you. If the waiting lasts a minute, then Oracle Database marks the session to be terminated and returns control to you with a message that the session is marked to
be terminated. The PMON background process then marks the session as terminated
when the activity is complete.