UNDO表空间的数据文件从OS直接删除了，数据库还没shutduwn，UNDO需要恢复。
1，数据库正常运行状态中；
2，rm - fundotbs02.dbf， 数据库在不知情的情况下接着运行，直到有日志错误告警。
3，此时创建spfile也会报错。
SQL> create pfile from spfile;
create pfile from spfile
*
ERROR at line 1:
ORA-01116: error in opening database file 7
ORA-01110: data file 7: '/u01/app/oracle/ORA11G/undotbs02.dbf'
ORA-27041: unable to open file
Linux Error: 2: No such file or directory
Additional information: 3
4，此时，注意保留现场，不要随意关闭数据库，查询有哪里UNDO的段正在使用。
SQL> select * from v$rollname;
USN NAME
---------- ------------------------------
0 SYSTEM
11 _SYSSMU11_2087506584$
12 _SYSSMU12_1903386335$
13 _SYSSMU13_3816552520$
14 _SYSSMU14_2728152801$
15 _SYSSMU15_2082928664$
16 _SYSSMU16_1938921541$
17 _SYSSMU17_3062294638$
8 rows selected.
如果数据库已经强行关闭了，可以查询如下。
SQL> select segment_name,status,tablespace_name from dba_rollback_segs where status='NEEDS RECOVERY';
SEGMENT_NAME STATUS TABLESPACE_NAME
------------------------------ ---------------- ------------------------------
11 _SYSSMU11_2087506584$ NEEDS RECOVERY UNDOTBS2
12 _SYSSMU12_1903386335$ NEEDS RECOVERY UNDOTBS2
13 _SYSSMU13_3816552520$ NEEDS RECOVERY UNDOTBS2
14 _SYSSMU14_2728152801$ NEEDS RECOVERY UNDOTBS2
15 _SYSSMU15_2082928664$ NEEDS RECOVERY UNDOTBS2
16 _SYSSMU16_1938921541$ NEEDS RECOVERY UNDOTBS2
17 _SYSSMU17_3062294638$ NEEDS RECOVERY UNDOTBS2