Oracle 如何强制刷新Buffer Cache

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:

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Oracle 10g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$bh; COUNT(*)
----------
14375SQL> select count(*) from x$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

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。