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:

Logging Data Changes

Savepoint

Transactions