分类:
2008-10-16 19:58:03
手动刷新Buffer Cache主要的原因是为了排除Cache对于的影响,以此促使重新执行物理访问。以下为具体的示例参考:
◆ 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> |