排除Cache影响手动强制刷新Buffer Cache

王朝other·作者佚名  2008-06-01
窄屏简体版  字體: |||超大  

手动刷新Buffer Cache主要的原因是为了排除Cache对于测试的影响,以此促使Oracle重新执行物理访问。以下为具体的示例参考:

◆Oracle 9i里,Oracle提供了一个内部事件,用来强制刷新Buffer Cache,它的语法如下:

alter session set events 'immediate trace name flush_cache level 1';

或者如下所示:

alter session set events = 'immediate trace name flush_cache';

◆同样我们也可以使用alter system系统级来设置。

alter system set events = 'immediate trace name flush_cache';

◆ Oracle 10g里,Oracle提供一个新的特性,可以通过以下的命令来刷新Buffer Cache:

alter system flush buffer_cache;

下面这个示例演示了刷新Cache后的作用:

(1)首先我们来创建测试表

SQL> create table t as select * from dba_objects;

Table created.

SQL> analyze table t compute statistics;

Table analyzed.

SQL> select blocks,empty_blocks from dba_tables

2 where table_name='T' and owner='SYS';

BLOCKS EMPTY_BLOCKS---------- ------------

78 1

表T共有79个Block.

(2)x$bh

SQL> select count(*) from x$bh;

COUNT(*)

----------

14375SQL>

select count(*) from x$bh where state=0;

-- state =0 is free

COUNT(*)

----------

13960SQL>

alter system set events = 'immediate trace name flush_cache';

System altered.

SQL> select count(*) from x$bh where state=0;

COUNT(*)

----------

14375

在这里你可以发现flush_cache以后,所有的Buffer都被标记为free。

(3)最后请留意flush_cache对于查询的影响

SQL> set autotrace trace stat

SQL> select count(*) from t;

Statistics

----------------------------------

0 recursive calls

0 db block gets

81 consistent gets

79 physical reads

0 redo size

....

SQL>

SQL> select count(*) from t;

Statistics-----------------------

0 recursive calls

0 db block gets

81 consistent gets

0 physical reads

0 redo size

....

SQL> alter system set events = 'immediate trace name flush_cache';

System altered.SQL> select count(*) from t;

Statistics

---------------------------------- 0 recursive calls

0 db block gets

81 consistent gets

79 physical reads

0 redo size

....

SQL>

 
 
 
免责声明:本文为网络用户发布,其观点仅代表作者个人观点,与本站无关,本站仅提供信息存储服务。文中陈述内容未经本站证实,其真实性、完整性、及时性本站不作任何保证或承诺,请读者仅作参考,并请自行核实相关内容。
 
 
© 2005- 王朝網路 版權所有 導航