我們知道Oracle10g在v$database檢視中引入了current_scn，這個SCN來自底層表，代表當前的SCN，在Oracle9i中我們可以透過dbms_flashback.get_system_change_number來獲得系統的SCN。我的版本是oracle11g的。

但是注意current_scn還是有所不同的，看一下一個查詢：
SQL> col current_scn for 99999999999999999
SQL> select current_scn from v$database;       CURRENT_SCN
------------------
           1941617

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941650

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941675

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941678

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941683

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941689

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941691

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941762

SQL> conn system/yang as sysdba
已連線。
SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941780

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941794

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941794

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941795

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941797

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941797

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941798

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941799

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941799

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941799

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941800

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941800

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941801

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941801

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941801

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941802

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941802

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941802

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

       CURRENT_SCN
------------------
           1941803

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941806

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941808

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941809

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941811

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941812

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941814

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941815

SQL> select current_scn from v$database;

       CURRENT_SCN
------------------
           1941817

我們看到current_scn的查詢會直接導致SCN的增進，而其他方式並不會，至少執行兩次時不會。也就是說在這裡的current_scn就像是一個Sequence一樣，查詢會導致增進。這也很好理解，v$database只能透過增進當前的SCN才能保證獲得的SCN是Current的。可是如果不查詢呢？這個值肯定是不會增長的。

這裡和蓋國強大師說點不一樣。

http://www.eygle.com/archives/2007/06/oracle10g_current_scn.html

有點疑惑？那位朋友能給個解答！先謝謝了。。

Oracle10g與oracle11g中current_scn計算差別

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