肃清Cache影响手动逼迫革新Buffer Cache

本源：赛迪网作者：Alizze

手动革新Buffer Cache次要的启事是为了肃清Cache关于测试的影响，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供应了一个外部事件，用来逼迫革新Buffer Cache，它的语法如下:

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

或许如下所示:

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

◆异常我们也可以运用alter system零碎级来设置。

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

◆ Oracle 10g里，Oracle供应一个新的特性，可以经由进程以下的呼吁来革新Buffer Cache:

alter system flush buffer_cache;

上面这个示例演示了革新Cache后的感化:

（1）起首我们来创立测试表

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今后，全数的Buffer都被标记为free。

（3）最初请细致flush_cache关于盘诘的影响

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>