you should be able to:
1、Explain DML and undo data generation
2、Monitor and administer undo data
3、Describe the difference between undo data and redo data
4、Configure undo retention
5、Guarantee undo retention
6、Use the Undo Advisor
Undo data is:
A copy of original, premodified data
Captured for every transaction that changes data
Retained at least until the transaction is ended
Used to support:
Rollback operations
Read-consistent queries
Oracle Flashback Query, Oracle Flashback Transaction, and Oracle Flashback Table
Recovery from failed transactions
有undo表空间，以段的方式存储.
Undo information is stored in undo segments, which are stored in an undo tablespace. Undo tablespaces:
Are used only for undo segments
Have special recovery considerations
May be associated with only a single instance
Require that only one of them be the current writable undo tablespace for a given instance at any given time
原文：http://blog.51cto.com/3938853/2162778