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types of undo segments(undo段类型)

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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