About Undo Data
When 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 necessary for the following reasons:
■ To undo any uncommitted changes made to the database if a rollback is necessary.
A rollback can be needed 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 while other changes may be occurring against the data. With
read consistency, a user session does not see uncommitted changes made 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 query 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.
... zobacz całą notatkę