oracle怎樣清空緩衝區快取____oracle

在Oracle9i裡，Oracle提供了一個內部事件，用以強制重新整理Buffer Cache，其文法為:

alter session set events 'immediate trace name flush_cache level 1';

或者:

alter session set events = 'immediate trace name flush_cache';

類似的也可以使用alter system系統級設定:

alter system set events = 'immediate trace name flush_cache';

在Oracle10g中，Oracle提供一個新的特性，可以通過如下命令重新整理Buffer Cache:

alter system flush buffer_cache;

我們通過實驗來看一下重新整理Cache的作用:

1.建立測試表

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個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

我們注意到flush_cache以後，所有Buffer都被標記為free.

3. 觀察flush_cache對於查詢的影響

SQL> set autotrace trace statSQL> 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>