The POST_TRANSACTION and IMMEDIATE clauses can be used together, but the documentation states that in this case the IMMEDIATE clause is ignored. The POST_TRANSACTION clause waits for ongoing transactions to complete before disconnecting the session, while the IMMEDIATE clause disconnects the session and ongoing transactions are rolled back immediately. SQL> ALTER SYSTEM DISCONNECT SESSION 'sid,serial#' IMMEDIATE SQL> ALTER SYSTEM DISCONNECT SESSION 'sid,serial#' POST_TRANSACTION The SID and SERIAL# values of the relevant session can be substituted into one of the following statements. The basic syntax is similar to the KILL SESSION command with the addition of the POST_TRANSACTION clause. Unlike the KILL SESSION command which asks the session to kill itself, the DISCONNECT SESSION command kills the dedicated server process (or virtual circuit when using Shared Sever), which is equivalent to killing the server process from the operating system. The ALTER SYSTEM DISCONNECT SESSION syntax is an alternative method for killing Oracle sessions. If the USED_UREC value is decreasing for the session in question you should leave it to complete the rollback rather than killing the session at the operating system level. You can do this by running this script ( session_undo.sql). Before doing this it's worth checking to see if it is performing a rollback. If the marked session persists for some time you may consider killing the process at the operating system level. This does not affect the work performed by the command, but it returns control back to the current session immediately, rather than waiting for confirmation of the kill. SQL> ALTER SYSTEM KILL SESSION 'sid,serial#' IMMEDIATE In addition to the syntax described above, you can add the IMMEDIATE clause. It will then be killed as soon as possible. In these cases the session will have a status of "marked for kill". In some situations, like waiting for a reply from a remote database or rolling back transactions, the session will not kill itself immediately and will wait for the current operation to complete. It merely asks the session to kill itself. SQL> ALTER SYSTEM KILL SESSION KILL SESSION command doesn't actually kill the session. This allows you to kill a session on different RAC node. In a RAC environment, you optionally specify the INST_ID, shown when querying the GV$SESSION view. SQL> ALTER SYSTEM KILL SESSION 'sid,serial#' The basic syntax for killing a session is shown below. Multithreaded Model using THREADED_EXECUTION in Oracle Database 12c Release 1 (12.1).ALTER SYSTEM CANCEL SQL : Cancel a SQL Statement in a Session in Oracle Database 18c.There are a number of ways to kill rogue sessions both within Oracle and externally. Home » Articles » Misc » Here Killing Oracle Sessions (ALTER SYSTEM KILL / DISCONNECT SESSION)