They won't be SEEN. but the instance recovery initiated by the SMON will do the following.................

Due to the way in which DBWn writes database buffers to datafiles(TARRY SAYS: here i would assume that the write ahead logging forced by sga flooded with subsequent queries,and limited space in the db buffer,will prompt the DBWn to start writing modifictions the last firsts ...how don't ask me this is what oracle says, maybe the experienced oracle guys can explain why that happens) being read at any given time a datafile may contain some tentative modifications by uncommitted transactions and may not contain some modifications by committed transactions. Therefore, two potential situations can result after a failure:

Data blocks containing committed modifications were not written to the datafiles, so the changes may only appear in the redo log. Therefore, the redo log contains committed data that must be applied to the datafiles.

Since the redo log may have contained data that was not committed, uncommitted transaction changes applied by the redo log during recovery must be erased from the datafiles.

To solve this situation, two separate steps are always used by Oracle during recovery from an instance or media failure: rolling forward and rolling back.

Rolling Forward
The first step of recovery is to roll forward, that is, reapply to the datafiles all of the changes recorded in the redo log. Rolling forward proceeds through as many redo log files as necessary to bring the datafiles forward to the required time.

If all necessary redo information is online, Oracle rolls forward automatically when the database starts. After roll forward, the datafiles contain all committed changes as well as any uncommitted changes that were recorded in the redo log.

Rolling Back
The roll forward is only half of recovery. After the roll forward, any changes that were not committed must be undone. After the redo log files have been applied, then the rollback segments are used to identify and undo transactions that were never committed, yet were recorded in the redo log. This process is called rolling back. Oracle completes this step automatically.


Hope that explains it.