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About Undo Data

Whcn a transaction modifies data, Oracle Database copies the original data before modifying it. The original copy of the modified data is called undo data. Saving this information is ncccssary for the following reasons:

■To undo any uncommitted changes madę to the database if a rollback is necessary. A rollback can be nceded because a user wants to undo the changes of a misguided or unintentional transaction, or it can be part of a recovery operation.

■To provide read consistency. which means that each user can get a consistent view of data. even whilc other changes may be occurring against the data. With read consistency, a user session does not see uncommitted changes madę in other user sessions (sometimes referred to as dirty reads). For example, if a user issues a query at 10:00 a.m. and the query lasts for 15 minutes, then the qucry results reflect the entire State of the data at 10:00 a.m.. regardless of update or insert operations performed by other users after the query started.

■To enable certain Oracle Flashback features, such as Oracle Flashback Query and Oracle Flashback Table, which enable you to view or recover data to a previous point in time.