I killed a session and it is still running?
From a thread on AskTom, when a session is killed:


  The session is killed but it is being cleaned up.  When you kill a session -- 
  outstanding work must still be cleaned up.


When you do DML on a table, Oracle may make the changes to the base table almost immediately, even if you don't commit. The other sessions don't see these changes yet thanks to multiversion read consistency but the actual physical block may already be overwritten with your uncommited work.

This is done because Oracle is optimized for commit. Most sessions should rarely rollback so Oracle anticipates by writing preemptively the changes. This is also one of the reasons that allows Oracle to have arbitrarily large transactions (limited only by the size of the undo tablespace). The changes are not delayed (much). This is also why you almost never wait for a commit in Oracle, even for multimillion-row DML.

One of the consequences is that rollback needs to undo the changes. The uncommited transaction is read back from the undo tablespace and each and every change is undone in reverse order. This can take longer than the initial DML for several reasons:


The rollback is not as well optimized, in particular multi-row operations can take a lot longer than regular DML.
Some changes are more expensive in reverse. For instance a delete which is a relatively cheap DML becomes an insert in reverse (which is more expensive).


This explains why rollback can take a lot of time. You have few means to accelerate the process. In any case you should not:


kill the rollback process
shutdown the database in the hope that this will terminate the session


If you shutdown the database normally (or transactional or immediate), Oracle will wait for the process to complete so that the database is left in a consistent state. If you shutdown ABORT, the database will be left in an inconsistent state (uncommited data written to disk) and Oracle will resume the rollback as soon as the database is restarted.

The rollback is handled by smon, I wouldn't try to kill it.



Some session, after being killed, will remain in the v$session view, even after all their changes have been entirely rolled back. This is often because Oracle is waiting for the client to return a relevant error message, such as:

ora-00028 your session has been killed


If the client itself has already left or the network communication has been severed, the killed session may stay indefinitely until you restart the instance. I don't think these sessions take resources.