When you open the database, the instance attempts to acquire one or more undo
tablespaces. You determine whether to operate in automatic undo management mode
or manual undo management mode at instance startup using the UNDO_MANAGEMENT
initialization parameter. The supported values are AUTO or MANUAL. If AUTO, then the
instance is started in automatic undo management mode. The default value is MANUAL.
■ If you use the undo tablespace method, then you are using automatic undo
management mode. This is recommended.
■ If you use the rollback segment method of managing undo space, then you are
using manual undo management mode.

回滚表空间的获取和管理
1. 当打开数据库时 , 实例需要获取一个或多个回滚表空间
2. 如果用户正使用撤销表空间 , 那么数据库就运行在自动撤销管理模式下 . Oracle 建议采用此种模式.
3. 如果用户正使用回滚段管理撤销空间,那么数据库就运行在手工撤销管理模式下 .