Example: Restart
A power outage has occurred and you have to restart the database. During the restart, the database system restores the database starting from the time of the last savepoint.
|
Transaction |
Description |
System Actions During the Restart |
|---|---|---|
|
T1 |
The transaction was started before an earlier savepoint, and finished with a COMMIT after an earlier savepoint (but before the system failure). |
None |
|
T2 |
The transaction was started before the second to last savepoint, but still running at the time of the system failure. |
The database system rolls back all changes that were made. To do this, it uses the corresponding undo log files. |
|
T3 |
The transaction was started before the last savepoint, but rolled back between the last savepoint and the system failure. |
The database system rolls back the changes that were made before the last savepoint. To do this, it uses the corresponding undo log files. |
|
T4 |
The transaction was started before the last savepoint, but finished with a COMMIT between the last savepoint and the system failure. |
The database system uses the required redo log entries for these transactions and repeats the transactions (ROLL FORWARD). |
|
T5 |
The transaction was started and rolled back between the last savepoint and the system failure. The system never wrote any changes to the data area. |
None |
|
T6 |
The transactions was started and finished with a COMMIT between the last savepoint and the system failure. |
The database system uses the required redo log entries for these transactions and repeats the transactions (ROLL FORWARD). |