Example: Restart 
A power outage has occurred and you have to restart the database instance (Restart). During the restart, the database system restores the database instance starting from the time of the last savepoint.
The following graphic shows how the database system handles the transactions when it is restarted.
Status of the Transactions at the Time of a System Failure
Handling of the Transactions at Restart
Transaction |
Action by the Database System |
T1 |
None The changes by transaction T1 were completely written to the data area by the last savepoint before the system failure. |
T2, T3 |
The database system rolls back the changes made before the savepoint. To do this, it evaluates the corresponding undo log files. |
T5 |
None The database system had already rolled back the changes by T5 before the system failure (ROLLBACK). These changes were never written to the data area through a savepoint. |
T4, T6 |
The database system imports the required redo log entries for these transactions and repeats the transactions (ROLL FORWARD). |
See also: