我们先来看一下这个现象：
[oracle@djp ora12]$ pwd
/u01/app/oracle/oradata/ora12
[oracle@djp ora12]$ mv undotbs01.dbf undotbs01.dbfbak
这里，我们把unto表空间对应的数据文件作了一个更改，表面该文件已经丢失。
下面，我们使用SQL*Plus，进行对数据库的启动：
idle> conn / as sysdba
Connected to an idle instance.
idle> startup mount;
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2293880 bytes
Variable Size 469766024 bytes
Database Buffers 356515840 bytes
Redo Buffers 6529024 bytes
Database mounted.
idle> alter database open;
alter database open
*
ERROR at line 1:
ORA-01157: cannot identify/lock data file 3 - see DBWR trace file
ORA-01110: data file 3: '/u01/app/oracle/oradata/ora12/undotbs01.dbf'
idle>
这时，发现启动失败。对于该现象， 我们可以进行如下的处理：
（1）设置undo_management为auto，由自动（auto）设置为手动（manual）。这因为，当设置为auto时，系统采用undo表空间管理回滚段，当设置manual时，系统使用回滚段。这里，相当于是禁用undo tablespace。
idle> alter system set undo_management = manual scope=spfile;
System altered.
idle>
（2）再次启动数据库，使用其生效
idle> shutdown immediate;
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.
idle> startup;
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2293880 bytes
Variable Size 444600200 bytes
Database Buffers 381681664 bytes
Redo Buffers 6529024 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 3 - see DBWR trace file
ORA-01110: data file 3: '/u01/app/oracle/oradata/ora12/undotbs01.dbf'
idle>
（3）对丢失的数据库文件进行一个离线删除，如下：
idle> alter database datafile '/u01/app/oracle/oradata/ora12/undotbs01.dbf'
2 offline drop
3 /
Database altered.
idle>
（4）打开数据库，并对undo tablespace重新创建。
idle> alter database open;
Database altered.
idle> drop tablespace undotbs
2 /
Tablespace dropped.
idle> create undo tablespace undotbs
2 datafile '/u01/app/oracle/oradata/ora12/undotbs01.dbf'
3 size 100M
4 autoextend on
5 next 50M
6 maxsize unlimited
7 /
Tablespace created.
idle>
（5）再次设置undo_management为auto，并重启数据库：
idle> alter system set undo_management = auto scope=spfile
2 /
System altered.
idle> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
idle> startup;
ORACLE instance started.
Total System Global Area 835104768 bytes
Fixed Size 2293880 bytes
Variable Size 469766024 bytes
Database Buffers 356515840 bytes
Redo Buffers 6529024 bytes
Database mounted.
Database opened.
idle> desc dba_objects;
Name Null? Type
----------------------------------------- -------- ----------------------------
OWNER VARCHAR2(128)
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(128)
OBJECT_ID NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(23)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
NAMESPACE NUMBER
EDITION_NAME VARCHAR2(128)
SHARING VARCHAR2(13)
EDITIONABLE VARCHAR2(1)
ORACLE_MAINTAINED VARCHAR2(1)
idle>
到此，该问题成功解决。