oracle学习笔记11_Managing Undo Data
--------Types of Undo Segments
system:Used for objects in the system tablespace.
Non-system:Used for objects in other tablespaces:
    auto mode:Requires an UNDO tablespace
    Manual mode:private & public
deferred:Used when tablespaces are taken offline immediate,temporary or for recovery.
--------Auto Undo Mgmt:concepts
Undo data is managed using an UNDO tablespace.
Allocate one UNDO tablespace per instance with enough space for the workload of the instance.
The oracle server automatically maintains undo data whtin the UNDO tablespace.
--------Configure two parameters in the initialization file:
UNDO_MANAGEMENT
UNDO_TABLESPACE
--------Switching UNDO tablespace
alter system set undo_tablespace=TABLESPACE;
--------
An undo tablespace can be only be dropped if it is currently not in use by any instance.
--------Information about undo segments
dba_rollback_segs
--------Dynamic performacne views
V$ROLLNAME
V$ROLLSTAT
V$UNDOSTAT
V$SESSION
V$TRANSACTION