Undo

Oracle Runtime Context

Oracle undo stores information needed to roll back transactions and reconstruct older row versions for read consistency. A reader can use undo to obtain the version appropriate to its snapshot SCN, while rollback uses related information to reverse uncommitted changes.

Recovery Boundary

Undo is not a backup and does not perform the same role as redo. Redo reapplies changes during recovery; undo provides the logical information required to return to an earlier row image or cancel a transaction.

Related Oracle Concepts

• Redo Log
• MVCC
• System Change Number
• Consistent Read

Source

• https://docs.oracle.com/en/database/oracle/oracle-database/19/cncpt/transactions.html