問：Oracle10g的current_scn是如何計(jì)算的? 答：我們知道Oracle10g在v$database視圖中引入了current_scn，這個(gè)SCN來自底層表，代表當(dāng)前的SCN，在Oracle9i中我們可以通過dbms_Flashback.get_system_change_number來獲得系統(tǒng)的SCN。 但是注重current_scn還是有所不同的，我們看一下一個(gè)查詢：oracle@danaly ~]$ 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$database;
CURRENT_SCN
------------------
8910961765230我們看到current_scn的查詢會(huì)直接導(dǎo)致SCN的增進(jìn)，而其他方式并不會(huì)。也就是說在這里的current_scn就像是一個(gè)Sequence一樣，查詢會(huì)導(dǎo)致增進(jìn)。這也很好理解，v$database只能通過增進(jìn)當(dāng)前的SCN才能保證獲得的SCN是Current的。可是假如不查詢呢？這個(gè)值肯定是不會(huì)增長的。也就是說你不查詢就不知道current_scn的值，只要查詢它就會(huì)出現(xiàn)變化。