[20121227]v$datafile訪問是從數據文件獲取信息嗎.txt

記得以前開始學習oracle的時候,被問及訪問v$datafile從哪里獲取信息,當時想都沒想,從數據文件.

雖然當時對方并沒有告訴答案,顯然認為水平不再同一檔次上.

直到有一次看一篇blog:

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%';

實際上當我拿這個問題問初學者,許多的回答跟我當時一樣.為了加強理解自己做一些例子來說明:

測試很簡單,關閉數據庫,將表空間users對應的數據文件改名,啟動數據庫到mount狀態,執行前面的腳本:

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

--如果v$datafile從數據文件獲得,已經文件改名,應該無法獲取信息,而現在可以查詢到,至少說明不是從數據文件獲取的.

--實際上數據文件的SCN是從v$datafile_header獲取的.

SQL> column name format a50SQL> select file#,status, CHECKPOINT_CHANGE#,name from v$datafile_header;FILE# STATUS? CHECKPOINT_CHANGE# NAME---------- ------- ------------------ --------------------------------------------------1 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/system01.dbf2 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/sysaux01.dbf3 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/undotbs01.dbf4 ONLINE?????????????????? 05 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/example01.dbf6 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/rman01.dbf7 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/tools01.dbf8 ONLINE????????? 3010736324 /u01/app/oracle11g/oradata/test/test01.dbf8 rows selected.

--如果你把數據文件全部移走.依舊可以訪問v$datafile視圖.

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?????????????? 30107363242????????? 1????????? 2 ONLINE? /u01/app/oracle11g/oradata/test/sysaux01.dbf?????????????? 30107363243????????? 2????????? 3 ONLINE? /u01/app/oracle11g/oradata/test/undotbs01.dbf????????????? 30107363244????????? 4????????? 4 ONLINE? /u01/app/oracle11g/oradata/test/users01.dbf??????????????? 30107363245????????? 6????????? 5 ONLINE? /u01/app/oracle11g/oradata/test/example01.dbf????????????? 30107363246????????? 7????????? 6 ONLINE? /u01/app/oracle11g/oradata/test/rman01.dbf???????????????? 30107363247????????? 8????????? 7 ONLINE? /u01/app/oracle11g/oradata/test/tools01.dbf??????????????? 30107363248????????? 9????????? 8 ONLINE? /u01/app/oracle11g/oradata/test/test01.dbf???????????????? 30107363248 rows selected.

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