Whenever a process or a user session changes data in the database, Oracle saves the old value as it existed before it was modified as undo data. This provides a number of benefits to the database user.
It lets the user change their mind and roll back, or undo, the change to the database.
It supports read-consistent queries. Once a query starts, any changes to the query's underlying tables are not reflected in the query's results.
It supports flashback query, an Oracle feature introduced in Oracle9 i. Flashback query allows a user to see how a table looked at some point in the past. As long as the undo data still exists for the requested point of time, flashback queries are possible.
In the following sections, we ...