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DB link与检查点(checkpoint)和SCN
作者：eygle |【转载时请务必以超链接形式标明文章原始出处和作者信息及本声明】链接：http://www.eygle.com/archives/2006/11/db_link_checkpoint_scn.html
昨天在测试的时候发现了一个有趣的现象，就是通过DB查询的2个数据库间，SCN会被同步。
测试的过程如下.
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 - 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 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:18992092078
scn of locald:18992092078
我们可以看到，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
这种机制其实是为了满足分布式事务(Distributed Transaction)的需要，只不过这里通过db link被触发。
-The End-
By eygle on 2006-11-08 11:48 | Comments (0) | Posted to Internal | Edit |Pageviews:
