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  • 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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  • 原文地址:https://www.cnblogs.com/zgqjymx/p/1974366.html
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