1. 启动SQLPLUS，并用sys登陆到数据库。
#su - oracle
$>sqlplus / as sysdba 2. 查找数据库的UNDO表空间名,确定当前例程正在使用的UNDO表空间：
Show parameter undo_tablespace。3. 确认UNDO表空间；
SQL> select name from v$tablespace;
NAME
------------------------------
UNDOTBS1 4. 检查数据库UNDO表空间占用空间情况以及数据文件存放位置；
select file_name, bytes / 1024 / 1024 / 1024from dba_data_fileswhere tablespace_name like 'UNDOTBS%';5. 查看回滚段的使用情况，哪个用户正在使用回滚段的资源，如果有用户最好更换时间（特别是生产环境）。
select s.username, u.namefrom v$transaction t, v$rollstat r, v$rollname u, v$session swhere s.taddr = t.addrand t.xidusn = r.usnand r.usn = u.usnorder by s.username;查询结果为空的话就能删除。6. 检查UNDO Segment状态；
select usn,xacts,rssize / 1024 / 1024 / 1024,hwmsize / 1024 / 1024 / 1024,shrinksfrom v$rollstatorder by rssize;USN  XACTS  RSSIZE/1024/1024/1024  HWMSIZE/1024/1024/1024  SHRINKS
8   0   0.00017547607421875 3.11521148681641    700
3   0   0.00023651123046875 3.22954559326172    632
0   0   0.00035858154296875 0.00035858154296875 0
1   0   0.00206756591796875 3.04867553710938    920
10  0   0.00206756591796875 0.648170471191406   819
7   0   0.00231170654296875 3.94835662841797    730
4   0   0.00304412841796875 2.00011444091797    651
11  0   0.00695037841796875 2.26921844482422    740
9   0   0.00792694091796875 2.07530975341797    773
6   0   0.00792694091796875 1.31906890869141    775
2   0   0.00890350341796875 3.13677215576172    699
5   0   1.96833801269531    3.99906921386719    267这还原表空间中还存在12个回滚的对象。7. 创建新的UNDO表空间，并设置自动扩展参数；
create undo tablespace undotbs2 datafile '/u02/pnrdb/undotbs2_01.dbf' size 100m reuse autoextend on next 100m maxsize unlimited;8. 切换UNDO表空间为新的UNDO表空间 ， 动态更改spfile配置文件；alter system set undo_tablespace=undotbs2 scope=both;  9.验证当前数据库的 UNDO表空间
SQL> show parameter undo
NAME                                 TYPE        VALUE
------------------------------------ ----------- --------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS2
9. 等待原UNDO表空间所有UNDO SEGMENT OFFLINE；
select usn,xacts,status,rssize / 1024 / 1024,hwmsize / 1024 / 1024,shrinksfrom v$rollstatorder by rssize;select usn,xacts,status,rssize / 1024 / 1024,hwmsize / 1024 / 1024,shrinksfrom v$rollstatorder by rssize;
select t.segment_name, t.tablespace_name, t.segment_id, t.statusfrom dba_rollback_segs t;SEGMENT_NAME      TABLESPACE_NAME SEGMENT_ID   STATUS
_SYSSMU1$   UNDOTBS1    1   OFFLINE
_SYSSMU2$   UNDOTBS1    2   OFFLINE
_SYSSMU3$   UNDOTBS1    3   OFFLINE
_SYSSMU4$   UNDOTBS1    4   OFFLINE
_SYSSMU5$   UNDOTBS1    5   OFFLINE
_SYSSMU6$   UNDOTBS1    6   OFFLINE
_SYSSMU7$   UNDOTBS1    7   OFFLINE
_SYSSMU8$   UNDOTBS1    8   OFFLINE
_SYSSMU9$   UNDOTBS1    9   OFFLINE
_SYSSMU10$  UNDOTBS1    10  OFFLINE
_SYSSMU11$  UNDOTBS1    11  OFFLINE
_SYSSMU12$  UNDOTBS1    12  OFFLINE
_SYSSMU13$  UNDOTBS1    13  OFFLINE
_SYSSMU14$  UNDOTBS1    14  OFFLINE
_SYSSMU15$  UNDOTBS1    15  OFFLINE
_SYSSMU16$  UNDOTBS1    16  OFFLINE
_SYSSMU17$  UNDOTBS1    17  OFFLINE
_SYSSMU18$  UNDOTBS1    18  OFFLINE
_SYSSMU19$  UNDOTBS1    19  OFFLINE
_SYSSMU20$  UNDOTBS1    20  OFFLINE
_SYSSMU21$  UNDOTBS1    21  OFFLINE
_SYSSMU22$  UNDOTBS1    22  OFFLINE
_SYSSMU23$  UNDOTBS1    23  OFFLINE
_SYSSMU24$  UNDOTBS1    24  OFFLINE
_SYSSMU25$  UNDOTBS1    25  OFFLINE
_SYSSMU26$  UNDOTBS1    26  OFFLINE
_SYSSMU27$  UNDOTBS1    27  OFFLINE
_SYSSMU28$  UNDOTBS1    28  OFFLINE
_SYSSMU29$  UNDOTBS1    29  OFFLINE
_SYSSMU30$  UNDOTBS1    30  OFFLINE
_SYSSMU31$  UNDOTBS1    31  OFFLINE
_SYSSMU32$  UNDOTBS1    32  OFFLINE
_SYSSMU33$  UNDOTBS1    33  OFFLINE
_SYSSMU34$  UNDOTBS1    34  OFFLINE
_SYSSMU35$  UNDOTBS1    35  OFFLINE上面对应的UNDOTBS1还原表空间所对应的回滚段均为OFFLINE10.到$ORACLE_HOME/dbs/init$ORACLE_SID.ora如下内容是否发生变更：
#cat $ORACLE_HOME/dbs/initddptest.ora
……
*.undo_management=’AUTO’
*.undo_retention=10800
*.undo_tablespace=’UNDOTBS2’
……如果没有发生变更请执行如下语句：
SQL> create pfile from spfile;
File created.11. 删除原有的UNDO表空间；drop tablespace undotbs1 including contents and datafiles;12. os级别释放undo数据文件；到root下执行
lsof |grep /u02/pnrdb/undotbs01.dbflsof |grep /u02/pnrdb/undotbs01.dbf |awk '{printf"kill -9 "$2"\n"}'