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【赛迪网－it技术报道】手动刷新buffer cache主要的原因是为了排除cache对于测试的影响，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提供了一个内部事件，用来强制刷新buffer cache，它的语法如下:
alter session set events 'immediate trace name flush_cache level 1';
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或者如下所示:
alter session set events = 'immediate trace name flush_cache';
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◆同样我们也可以使用alter system系统级来设置。
alter system set events = 'immediate trace name flush_cache';
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◆ oracle 10g里，oracle提供一个新的特性，可以通过以下的命令来刷新buffer cache:
alter system flush buffer_cache;
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下面这个示例演示了刷新cache后的作用:
（1）首先我们来创建测试表
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
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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
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在这里你可以发现flush_cache以后，所有的buffer都被标记为free。
（3）最后请留意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>
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