今天做了一下undo隐藏参数的实验

在没有备份的情况下，删除正在使用的undo,然后关机

(本次使用的的oracle的隐藏参数，慎用！！！！！！！！！！！！！！)

idle> select * from V$VERSION;


BANNER
----------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

sys@DNDN> startup

ORACLE instance started.


Total System Global Area  335544320 bytes
Fixed Size    2020640 bytes
Variable Size  117443296 bytes
Database Buffers  213909504 bytes
Redo Buffers    2170880 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 2 - see DBWR trace file
ORA-01110: data file 2: '/u01/app/oracle/oradata/dndn/undotbs1.dbf'




sys@DNDN> show parameter undo_t


NAME     TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_tablespace     stringUNDOTBS1

在此之前我有建立有一个未使用的undo 名字叫undotbs(区别上面的undotbs1)（*注：若没有多余的undo,则见文章最后解决办法）

sys@DNDN>shutdown immediate;

idle> startup nomount;
ORACLE instance started.


Total System Global Area  335544320 bytes
Fixed Size    2020640 bytes
Variable Size  117443296 bytes
Database Buffers  213909504 bytes
Redo Buffers    2170880 bytes

#把undo_tablespace 修改成备用的undo

idle>alter system set undo_tablespace=undotbs scope=spfile;

#把undo改成手动管理

idle> alter system set undo_management=manual scope=spfile; 

System altered.

#使用隐藏参数
idle> alter system set "_offline_rollback_segments"=true scope=spfile;


System altered.

idle> shutdown immediate;

idle> startup
ORACLE instance started.


Total System Global Area  335544320 bytes
Fixed Size    2020640 bytes
Variable Size  117443296 bytes
Database Buffers  213909504 bytes
Redo Buffers    2170880 bytes
Database mounted.
Database opened.

这样数据库就可以打开了！

查看UNDOTBS1  (之前丢失的表空间) 中的段的状态是否都是offline

idle> select segment_name,tablespace_name,status from dba_rollback_segs;


SEGMENT_NAME        TABLESPACE_NAME      STATUS
------------------------------ ------------------------------ ----------------
SYSTEM        SYSTEM      ONLINE
_SYSSMU1$        UNDOTBS1       OFFLINE
_SYSSMU2$