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what is SMON_SCN_TIME ?
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/2005/12/what_is_smon_scn_time.html
SMON_SCN_TIME是Oracle数据库的系统表，用以进行辅助恢复等功能。
在Oracle9iR2中,SMON_SCN_TIME每5分钟被更新一次。
[oracle@jumper oracle]$ sqlplus "/ as sysdba" SQL*Plus: Release 9.2.0.4.0 - Production on Mon Dec 19 18:19:27 2005 Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
SQL> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss'; Session altered. SQL> select * from smon_scn_time where rownum <10; THREAD TIME_MP TIME_DP SCN_WRP SCN_BAS 9 rows selected. |
SMON_SCN_TIME记录5天的数据，也就是1440 ((5d x 24h x 12m = 1440) 条记录。
所以在Oracle9iR2中，表属性修改时间和flashback时间差至少应为5分钟。否则就会出现ORA-01466错误。
[oracle@jumper oracle]$ sqlplus eygle/eygle SQL*Plus: Release 9.2.0.4.0 - Production on Mon Dec 19 19:34:53 2005 Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
19:34:58 SQL> col myscn for 9999999999999999 Table created. 19:35:00 SQL> exec :myscn := dbms_flashback.get_system_change_number(); PL/SQL procedure successfully completed. 19:35:00 SQL> print myscn; MYSCN 19:35:00 SQL> insert into eygle values(1); 1 row created. 19:35:00 SQL> commit; Commit complete. 19:35:00 SQL> exec :myscn := dbms_flashback.get_system_change_number(); PL/SQL procedure successfully completed. 19:35:00 SQL> print myscn; MYSCN 19:35:00 SQL> select * from eygle as of scn(:myscn); 19:41:10 SQL> select * from eygle as of scn(:myscn); 19:43:03 SQL> select dbms_flashback.get_system_change_number myscn from dual; MYSCN 19:43:15 SQL> select * from eygle as of scn 107374274944; C1 |
此时SMON_SCN_TIME中的信息如下:
19:45:11 SQL> select * from (select * from smon_scn_time order by time_dp desc) where rownum <5; THREAD TIME_MP TIME_DP SCN_WRP SCN_BAS |
在Oracle10g中，SMON_SCN_TIME表每6秒被更新一次。所以这个Flashback时间被缩小为6秒。
[oracle@danaly ~]$ sqlplus eygle/eygle SQL*Plus: Release 10.2.0.1.0 - Production on Mon Dec 19 19:37:19 2005 Copyright (c) 1982, 2005, Oracle. All rights reserved.
19:37:22 SQL> col myscn for 9999999999999999 Table created. 19:37:22 SQL> exec :myscn := dbms_flashback.get_system_change_number(); PL/SQL procedure successfully completed. 19:37:22 SQL> print myscn; MYSCN 19:37:22 SQL> insert into eygle values(1); 1 row created. 19:37:22 SQL> commit; Commit complete. 19:37:22 SQL> exec :myscn := dbms_flashback.get_system_change_number(); PL/SQL procedure successfully completed. 19:37:22 SQL> print myscn; MYSCN 19:37:22 SQL> select * from eygle as of scn(:myscn);
SCN_TO_TIMESTAMP(16011230124) 19:38:50 SQL> select timestamp_to_scn('19-DEC-05 07.37.27.000000000 PM') MYSCN from dual;
MYSCN 19:39:02 SQL> select * from eygle as of scn 16011230125; C1 |
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By eygle on 2005-12-27 20:19 | Comments (2) | Posted to Internal | Edit |Pageviews:
留言 (2)
所以在Oracle9iR2中，表属性修改时间和flashback时间差至少应为5分钟。否则就会出现ORA-01466错误
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Hi, eygle,你说的表属性修改时间是指什么时间呢？LAST_DDL_TIME吗？下面是我在9205下的试验，修改了表的属性后立刻flashback query,但没有报错：
SQL> alter table TEST20 add name varchar2(20);
Table altered.
SQL> exec :myscn := dbms_flashback.get_system_change_number();
PL/SQL procedure successfully completed.
SQL> print myscn;
MYSCN
-----------------
8174383932257
SQL> select * from test20 as of scn(:myscn);
ID NAME
---------- --------------------
1
SQL> insert into test20 values(1,'abcd');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test20 as of scn(:myscn);
ID NAME
---------- --------------------
1
Posted by: waityou81 at September 10, 2007 5:04 PM
不知道eygle大师是基于什么版本的测试，我在10.1.0.2下测试和楼上的那位兄弟一样。。咋好像没有时间限制，都可以恢复。
Posted by: Hopewell_Go at September 25, 2007 3:11 PM
