排除Cache影响手动强制刷新Buffer Cache

  来源：赛迪网    作者：Alizze

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>