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>

以上说明仅供参考。