查看操作系统：
SQL> !cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.4 (Maipo)
查看数据库版本：
SQL> set linesize 400
SQL> select * from v$version; BANNER CON_ID
---------------------------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production 0
PL/SQL Release 12.1.0.2.0 - Production 0
CORE 12.1.0.2.0 Production 0
TNS for Linux: Version 12.1.0.2.0 - Production 0
NLSRTL Version 12.1.0.2.0 - Production 0
查看当前使用的undo表空间信息：
SQL> show parameter undo_tablespace NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_tablespace string UNDOTBS1
SQL> col FILE_NAME format a100
SQL> select tablespace_name, file_id, file_name,round (bytes / (1024 * 1024), 0) total_space from dba_data_files where tablespace_name='UNDOTBS1'; TABLESPACE_NAME FILE_ID FILE_NAME TOTAL_SPACE
-------------------- ---------- ---------------------------------------------------------------------------------------------------- -----------
UNDOTBS1 4 /u01/app/oracle/oradata/orcl/undotbs01.dbf 220
1、数据库状态静止时（无DML操作期间）执行UNDO表空间切换（由UNDOTBS1切换为UNDOTBS2）
（1）创建新的undo表空间UNDOTBS2
SQL> create undo tablespace UNDOTBS2 datafile '/u01/app/oracle/oradata/orcl/undotbs02.dbf' size 10M; Tablespace created.
（2）切换UNDOTBS2为新的undo表空间
SQL> alter system set undo_tablespace = undotbs2 scope=both; System altered.
（3）此时数据库处于静止状态，无任何DML操作，查看UNDOTBS1已经处于OFFLINE状态
SQL> select tablespace_name , status , count(*) from dba_rollback_segs group by tablespace_name , status; TABLESPACE_NAME STATUS COUNT(*)
-------------------- ---------------- ----------
UNDOTBS1 OFFLINE 10
SYSTEM ONLINE 1
UNDOTBS2 ONLINE 11
（4）检查确认UNDOTBS1中没有ONLINE的segment
SQL> select status,segment_name from dba_rollback_segs where status not in ('OFFLINE') and tablespace_name='UNDOTBS1';
no rows selected（5）删除旧的UNDOTBS1
SQL> Drop tablespace UNDOTBS1 including contents and datafiles; Tablespace dropped.
（6）至此，undo表空间由UNDOTBS1成功切换为UNDOTBS2.
2、数据库中有DML操作期间，切换UNDO表空间步骤（由UNDOTBS2切换为UNDOTBS1）
SQL> create undo tablespace UNDOTBS1 datafile '/u01/app/oracle/oradata/orcl/undotbs01.dbf' size 220M; Tablespace created. SQL> select tablespace_name, file_id, file_name,round (bytes / (1024 * 1024), 0) total_space from dba_data_files; TABLESPACE_NAME FILE_ID FILE_NAME TOTAL_SPACE
-------------------- ---------- ---------------------------------------------------------------------------------------------------- -----------
SYSTEM 1 /u01/app/oracle/oradata/orcl/system01.dbf 790
SYSAUX 3 /u01/app/oracle/oradata/orcl/sysaux01.dbf 730
USERS 6 /u01/app/oracle/oradata/orcl/users01.dbf 5
UNDOTBS2 11 /u01/app/oracle/oradata/orcl/undotbs02.dbf 10
UNDOTBS1 12 /u01/app/oracle/oradata/orcl/undotbs01.dbf 220
（1）设置UNDOTBS1为默认undo表空间
SQL> alter system set undo_tablespace = undotbs1 scope=both; System altered.
（2）此时检查UNDOTBS1处于ONLINE状态
SQL> select tablespace_name , status , count(*) from dba_rollback_segs group by tablespace_name , status; TABLESPACE_NAME STATUS COUNT(*)
-------------------- ---------------- ----------
UNDOTBS1 ONLINE 11
SYSTEM ONLINE 1
UNDOTBS2 OFFLINE 11
（3）删除
SQL> select status,segment_name from dba_rollback_segs where status not in ('OFFLINE') and tablespace_name='UNDOTBS2';
no rows selected
SQL> Drop tablespace UNDOTBS2 including contents and datafiles;
Tablespace dropped.
SQL> show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
temp_undo_enabled boolean FALSE
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS1
SQL> （4）至此，undo表空间由UNDOTBS2又成功切换为UNDOTBS1