types of undo segments(undo段类型)

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从9i开始系统已经用undo替换掉了以前版本中的rollback,两种名称的意义是相同的，虽然现在改成了undo 不过，在数据字典查询中依然是去dba_rollback_segs中去查询；

有三种类型
 
     1，system 回滚段，   它是系统本身自己用的回滚段，保留在system tablespace 表空间中

    2，non-system回滚段   非系统的回滚段同时又分为两种情况
                         a) auto mode     是保留在系统参数中指定的 UNDO tablespace表空间中。
                         b) manul mode  又分为两类
                                          — private    single instance
                                          — public    RAC any instance

    3,deferrd    与system 回滚段一样也不需要人为的干涉，用于在把tablespace offline 到非normal,及immediate、temproray、for recovery时，再把tablespace online时
 做介质恢复里使用，因为这三种情况并没有把所有数据文件checkpoint;

与undo 设置相关的参数

undo_managment =  auto|manul
自动管理还时手动管理
undo_tablespace = xxx
当配置为自动管理时，有且只能激活一个UNDO TABLESPACE；
undo_supperess_errors = true|false    ———9i中的参数，在10已不存在
当在自动管理的mode下，手动建立rollback segment时，是否忽略这个错误操作
undo_retention
配置即使当你commit后，undo会保留的时间，用于回闪

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