1] If the session that initiated the transaction fails {n/w down , user process reboots} – PMON will rollback.
2] If the Server reboots then on startup SMON will detect and rollback.
3] Mechanism:
Update: The pre-update version is the columns are copied from the undo block to table blocks.
4] Insert: Oracle retrieves the rowid of the inserted row from the undo block and uses it as the key for a delete on a table.
5] Delete: Oracle constructs a complete insert statement from the data in the undo block.
Note: A Rollback will itself generate more redo as it executes , perhaps rather more than the original statement.
