Oracle HowTo:如何强制刷新Buffer Cache

很多时候，为了排除Cache对于测试的影响，我们常常需要手动刷新Buffer Cache，以促使Oracle重新执行物理访问。我曾经在使用 db_file_multiblock_read_count测试Oracle在不同系统中的IO能力一文中用到这个方法，在Itpub上也有朋友问到，今天整理一下并作简要说明。

在Oracle9i里，Oracle提供了一个内部事件，用以强制刷新Buffer Cache，其语法为:

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

或者:

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

类似的也可以使用alter system系统级设置:

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

在Oracle10g中，Oracle提供一个新的特性，可以通过如下命令刷新Buffer Cache:

alter system flush buffer_cache;

我们通过试验来看一下刷新Cache的作用:

1.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(*)
----------
     14375

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

  COUNT(*)
----------
     13960

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

System altered.

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

  COUNT(*)
----------
     14375

我们注意到flush_cache以后，所有Buffer都被标记为free.

3. 观察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>