Oracle10g的current_scn是如何计算的?

文章作者 100test 发表时间 2007:09:25 13:18:59
来源 100Test.Com百考试题网

Oracle10g的current_scn是如何计算的?

　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。

　但是注意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> 0select current_scn from v$database. CURRENT_SCN ------------------ 8910961765228 SQL> 0select dbms_flashback.get_system_change_number current_scn from dual. CURRENT_SCN ------------------ 8910961765228 SQL> 0select dbms_flashback.get_system_change_number current_scn from dual. CURRENT_SCN ------------------ 8910961765228 SQL> 0select current_scn from v$database. CURRENT_SCN ------------------ 8910961765229 SQL> 0select dbms_flashback.get_system_change_number current_scn from dual. CURRENT_SCN ------------------ 8910961765229 SQL> 0select dbms_flashback.get_system_change_number current_scn from dual. CURRENT_SCN ------------------ 8910961765229 SQL> 0select 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。可是如果不查询呢？这个值肯定是不会增长的。