Oracle 如何强制刷新Buffer Cache
在Oracle9i里，Oracle提供了一个内部事件，用以强制刷新Buffer Cache，其语法为:
alter session set events 'immediate trace name flush_cache level 1';
或者:
alter session set events = 'immediate trace name flush_cache';
类似的也可以使用alter system系统级设置:
alter system set events = 'immediate trace name flush_cache';
在Oracle 10g中，Oracle提供一个新的特性，可以通过如下命令刷新Buffer Cache:
alter system flush buffer_cache;
我们通过试验来看一下刷新Cache的作用:
1.创建测试表
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
我们注意到flush_cache以后，所有Buffer都被标记为free.
3. 观察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>
以上说明仅供参考。
