[20121227]v$datafile访问是从数据文件获取信息吗.txt

记得以前开始学习oracle的时候,被问及访问v$datafile从哪里获取信息,当时想都没想,从数据文件.
虽然当时对方并没有告诉答案,显然认为水平不再同一档次上.

直到有一次看一篇blog:
http://avdeo.com/2008/05/29/oracle-database-recovery-details/

SELECT 'controlfile' "SCN location", 'SYSTEM checkpoint' NAME, checkpoint_change#
  FROM v$database
UNION
SELECT 'file in controlfile', NAME, checkpoint_change#
  FROM v$datafile
 WHERE NAME LIKE '%users01%'
UNION
SELECT 'file header', NAME, checkpoint_change#
  FROM v$datafile_header
 WHERE NAME LIKE '%users01%';

实际上当我拿这个问题问初学者,许多的回答跟我当时一样.为了加强理解自己做一些例子来说明:


测试很简单,关闭数据库,将表空间users对应的数据文件改名,启动数据库到mount状态,执行前面的脚本:
$ mv users01.dbf users01.dbf.xxx

SQL> startup mount
ORACLE instance started.

Total System Global Area 2137886720 bytes
Fixed Size                  2215064 bytes
Variable Size            1728054120 bytes
Database Buffers          402653184 bytes
Redo Buffers                4964352 bytes
Database mounted.

SQL> column name format a50
SQL> SELECT 'controlfile' "SCN location", 'SYSTEM checkpoint' NAME, checkpoint_change#
UNION
SELECT 'file header', NAME, checkpoint_change#
  FROM v$database
UNION
SELECT 'file in controlfile', NAME, checkpoint_change#
  FROM v$datafile
 WHERE NAME LIKE '%users01%'
UNION
SELECT 'file header', NAME, checkpoint_change#
  FROM v$datafile_header
WHERE NAME LIKE '%users01%';

SCN location        NAME                                               CHECKPOINT_CHANGE#

------------------- -------------------------------------------------- ------------------

controlfile         SYSTEM checkpoint                                          3010736324

file in controlfile /u01/app/oracle11g/oradata/test/users01.dbf                3010736324

--如果v$datafile从数据文件获得,已经文件改名,应该无法获取信息,而现在可以查询到,至少说明不是从数据文件获取的.

--实际上数据文件的SCN是从v$datafile_header获取的.

SQL> column name format a50

SQL> select file#,status, CHECKPOINT_CHANGE#,name from v$datafile_header;

     FILE# STATUS  CHECKPOINT_CHANGE# NAME

---------- ------- ------------------ --------------------------------------------------

         1 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/system01.dbf

         2 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/sysaux01.dbf

         3 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/undotbs01.dbf

         4 ONLINE                   0

         5 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/example01.dbf

         6 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/rman01.dbf

         7 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/tools01.dbf

         8 ONLINE          3010736324 /u01/app/oracle11g/oradata/test/test01.dbf

8 rows selected.

--如果你把数据文件全部移走.依旧可以访问v$datafile视图.

SQL> select file#,TS#, RFILE#, STATUS ,NAME, CHECKPOINT_CHANGE# from v$datafile ;

     FILE#        TS#     RFILE# STATUS  NAME                                               CHECKPOINT_CHANGE#

---------- ---------- ---------- ------- -------------------------------------------------- ------------------

         1          0          1 SYSTEM  /u01/app/oracle11g/oradata/test/system01.dbf               3010736324

         2          1          2 ONLINE  /u01/app/oracle11g/oradata/test/sysaux01.dbf               3010736324

         3          2          3 ONLINE  /u01/app/oracle11g/oradata/test/undotbs01.dbf              3010736324

         4          4          4 ONLINE  /u01/app/oracle11g/oradata/test/users01.dbf                3010736324

         5          6          5 ONLINE  /u01/app/oracle11g/oradata/test/example01.dbf              3010736324

         6          7          6 ONLINE  /u01/app/oracle11g/oradata/test/rman01.dbf                 3010736324

         7          8          7 ONLINE  /u01/app/oracle11g/oradata/test/tools01.dbf                3010736324

         8          9          8 ONLINE  /u01/app/oracle11g/oradata/test/test01.dbf                 3010736324

8 rows selected.