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>
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