In a ROLLBACK, all the changes made by a transaction or a subtransaction on the database instance are reversed.
· Changes closed with a COMMIT can no longer be reversed with a ROLLBACK.
· As a result of a ROLLBACK, a new transaction is implicitly opened.
In normal database operation, the database system performs the required ROLLBACK actions independently. However, ROLLBACK can also be explicitly requested using appropriate SQL statements.
In a restart, the system checks which transactions were canceled or closed with a ROLLBACK. The actions are these transactions are undone.