Logical Structure of Undo Segments in the Database
Undo segments are used to save the old value of a changed data element whenever a process changes any data in the database. It stores the location of the data as well as the value of the data as it existed before it was modified.
The header of an undo segment contains an internal transaction table where the information about the current transaction using the undo segment gets stored. A serial transaction uses a single undo segment in which you can store all the undo information for that transaction, and several concurrent transactions can write to the same undo segment.
Oracle uses undo segments for several purposes. Those purposes are covered in the following section.
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