Oracle 如何強制重新整理Buffer Cache

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

以上說明僅供參考。