1 Oracle使用还原（回滚段）目的：
 1）事务回滚
 2）事务恢复
 3）保证数据的读一致性

2 查还原段是否自启动，所用表空间，保留的时间（秒）
SQL> l
  1  select name,value from v$parameter
  2* where name like '%undo%'
SQL> /

NAME                           VALUE
------------------------------ ------------------------------
undo_management                AUTO
undo_tablespace                UNDOTBS1
undo_retention                 900

3 查所有的还原表空间
SQL> select tablespace_name,status,contents
  2  from dba_tablespaces
  3  where contents='UNDO';

TABLESPACE_NAME                STATUS    CONTENTS
------------------------------ --------- ---------
UNDOTBS1                       ONLINE    UNDO

4 创建还原表空间
先查询数据文件所在位置
SQL> select name from v$datafile;

NAME
------------------------------
G:\ORACLE\PRODUCT\10.2.0\ORADA
TA\JXFGW\SYSTEM01.DBF

G:\ORACLE\PRODUCT\10.2.0\ORADA
TA\JXFGW\UNDOTBS01.DBF

G:\ORACLE\PRODUCT\10.2.0\ORADA
TA\JXFGW\SYSAUX01.DBF

G:\ORACLE\PRODUCT\10.2.0\ORADA
TA\JXFGW\USERS01.DBF
再创建还原表空间
SQL> create undo tablespace jinlian_undo
  2  datafile 'G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\jinlian_undo.dbf'
  3  size 20M;

表空间已创建。

5 查询是否创建好
SQL> select tablespace_name,status,contents
  2  from dba_tablespaces
  3  where contents='UNDO';

TABLESPACE_NAME                STATUS    CONTENTS
------------------------------ --------- ---------
UNDOTBS1                       ONLINE    UNDO
JINLIAN_UNDO                   ONLINE    UNDO

6 为还原表空间增加一个数据文件
SQL> alter tablespace jinlian_undo
  2  add datafile 'G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\jinlian2_undo.dbf'
  3  size 10M;

表空间已更改。

7 查还原表空间所对应的数据文件
SQL> l
  1  select file_id,file_name,tablespace_name,bytes/1024/1024 MB
  2  from dba_data_files
  3*   where tablespace_name like 'JIN%'
SQL> /

   FILE_ID FILE_NAME
---------- -------------------------------------------------------
TABLESPACE_NAME                        MB
------------------------------ ----------
         8 G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN_UNDO.DBF
JINLIAN_UNDO                           20

         9 G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN2_UNDO.DB
           F
JINLIAN_UNDO                           10

8 查还原表空间的数据文件是否可以自动扩展
SQL> l
  1  select file_id,file_name,tablespace_name,autoextensible
  2  from dba_data_files
  3*   where tablespace_name like 'JIN%'
SQL> /

   FILE_ID FILE_NAME
---------- -------------------------------------------------------
TABLESPACE_NAME                AUT
------------------------------ ---
         8 G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN_UNDO.DBF
JINLIAN_UNDO                   NO

         9 G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN2_UNDO.DB
           F
JINLIAN_UNDO                   NO

9 把数据文件改成自动扩展
SQL> alter database
  2  datafile 'G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN2_UNDO.DBF'
  3  autoextend on;

数据库已更改。

10 查询是否自动扩展
SQL> select file_id,tablespace_name,file_name,autoextensible
  2  from dba_data_files
  3  where tablespace_name like 'JIN%';

   FILE_ID TABLESPACE_NAME
---------- ------------------------------
FILE_NAME                                               AUT
------------------------------------------------------- ---
         8 JINLIAN_UNDO
G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN_UNDO.DBF NO

         9 JINLIAN_UNDO
G:\ORACLE\PRODUCT\10.2.0\ORADATA\JXFGW\JINLIAN2_UNDO.DB YES
F