测试的具体过程：
1.首先获得本地的SCN
[oracle@jumper oracle]$ sqlplus '/ as sysdba'SQL*Plus: Release 9.2.0.4.0 - Production on Tue Nov 7 21:07:56 2006Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.Connected to:Oracle9i Enterprise Edition Release 9.2.0.4.0 - ProductionWith the Partitioning optionJServer Release 9.2.0.4.0 - ProductionSQL> select dbms_flashback.GET_SYSTEM_CHANGE_NUMBER scn from dual;SCN----------5287824
2.通过db link进行SCN查询
SQL> set serveroutput onSQL> set feedback offSQL> declare 2 r_gname varchar2(40); 3 l_gname varchar2(40); 4 begin 5 execute immediate 6 'select GLOBAL_NAME from global_name@hsbill' into r_gname; 7 dbms_output.put_line('gname of remote:'||r_gname); 8 select GLOBAL_NAME into l_gname from global_name; 9 dbms_output.put_line('gname of locald:'||l_gname); 10 end; 11 /gname of remote:HSBILL.HURRAY.COM.CNgname of locald:EYGLESQL> declare 2 r_scn number; 3 l_scn number; 4 begin 5 execute immediate 6 'select dbms_flashback.GET_SYSTEM_CHANGE_NUMBER@hsbill from dual' into r_scn; 7 dbms_output.put_line('scn of remote:'||r_scn); 8 select dbms_flashback.GET_SYSTEM_CHANGE_NUMBER into l_scn from dual; 9 dbms_output.put_line('scn of locald:'||l_scn); 10 end; 11 /scn of remote:18992092078scn of locald:18992092078
大家可以看到，通过DB Link查询后，两个数据库的SCN被同步。
手工执行checkpoint，此时可以发现数据库的checkpoint scn被增进:
SQL> col scn for 999999999SQL> select file#,CHECKPOINT_CHANGE# scn from v$datafile; FILE# SCN---------- ----------------- 1 5287605 2 5287605 3 5287605 4 5287605SQL> alter system checkpoint;System altered.SQL> select file#,CHECKPOINT_CHANGE# scn from v$datafile; FILE#SCN---------- ----------------- 1 18992092162 2 18992092162 3 18992092162 4 18992092162
以上的这种机制其实是为了满足分布式事务(Distributed Transaction)的需要，只不过在此处是通过db link被触发的。
