通过DB查询的两个数据库间SCN会被同步

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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。