In Oracle "undo" and "redo" are 2 different concepts."Undo" refers to rollback segment and "redo" refers to redo log files.Whenever Oracle instance crashes,Oracle performs Instance recovery using Undo and Redo methodology.
Undo contains all the transaction entries which are not yet commited in the database.Any change made to the data,its previous value is stored in the rollback segement as well as redo log file.So in case a user rollbacks the transcation,the previous value is stored back in the table.
Redo contains both pre-image and post-image.Redo log files contains all the changes made to the database buffer cache.It is also used in recovery purpose to bring the database to the point of failure.
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