Undoing (rolling
back) a transaction means
that the transaction is reset, that is to say, the database is set to the
consistent state that it had before the transaction was started. The undo
measure means that additional, redundant data management is required.
Therefore, for each transaction, the original values of the data objects that are to be changed by the transaction, that is, the values that were available before the transaction was started, are stored. These logged values are described as undo log entries (or before-image entries).
Undo log entries
are permanently stored in the stored in the data area before
an SQL statement is executed (Undo Log
Management). Each transaction can access its undo log entries without any
locks being set.
In addition, the undo log entries are needed for the undo if the database is restarted or recovered and are used for History Management.
See also: Redo Log Entry