通过DB查询的2个数据库间，SCN会被同步:
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 2006
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
SQL> 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 on
SQL> set feedback off
SQL> 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.CN
gname of locald:EYGLE
SQL> 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 999999999
SQL> select file#,CHECKPOINT_CHANGE# scn from v$datafile;
FILE# SCN
---------- -----------------
1 5287605
2 5287605
3 5287605
4 5287605
SQL> alter system checkpoint;
System altered.
SQL> select file#,CHECKPOINT_CHANGE# scn from v$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。
