Oracle10g中的current_scn是如何计算的

问：Oracle10g的current_scn是如何计算的?

答：我们知道Oracle10g在v$database视图中引入了current_scn，这个SCN来自底层表，代表当前的SCN，在Oracle9i中我们可以通过dbms_flashback.get_system_change_number来获得系统的SCN。

但是注重current_scn还是有所不同的，我们看一下一个查询：

oracle@danaly ~]$ sqlplus '/ as sysdba'

SQL*Plus: Release 10.2.0.1.0 - ProdUCtion on

Thu Jun 21 10:15:08 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to:

Oracle Database 10g Enterprise Edition

Release 10.2.0.1.0 - Production

With the Partitioning, Oracle Label Security,

OLAP and Data Mining Scoring Engine options

SQL> @scn

SQL> col current_scn for 99999999999999999

SQL> select current_scn from v$database;

CURRENT_SCN

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

8910961765228

SQL> select dbms_flashback.get_system_change_number

current_scn from dual;

CURRENT_SCN

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

8910961765228

SQL> select dbms_flashback.get_system_change_number

current_scn from dual;

CURRENT_SCN

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

8910961765228

SQL> select current_scn from v$database;

CURRENT_SCN

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

8910961765229

SQL> select dbms_flashback.get_system_change_number

current_scn from dual;

CURRENT_SCN

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

8910961765229

SQL> select dbms_flashback.get_system_change_number

current_scn from dual;

CURRENT_SCN

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

8910961765229

SQL> select current_scn from v$database;

CURRENT_SCN

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

8910961765230

我们看到current_scn的查询会直接导致SCN的增进，而其他方式并不会。也就是说在这里的current_scn就像是一个Sequence一样，查询会导致增进。这也很好理解，v$database只能通过增进当前的SCN才能保证获得的SCN是Current的。可是假如不查询呢？这个值肯定是不会增长的。

也就是说你不查询就不知道current_scn的值，只要查询它就会出现变化。