【转】Cache Buffer Chain 第三篇

 文章转自：http://oracle.chinaitlab.com/induction/862509.html，文章前部分转载，后部分自己加上的。

Oracle数据库只读模式的CACHE BUFFERS CHAINS测试是本文我们主要要介绍的内容,虽然从Oracle 9i里边引入了只读模式的CACHE BUFFERS CHAINS,但是在获取BUFFER LOCK的时候,还是会请求CACHE BUFFERS CHAINS,但是情形没有8i里边那么严重了,接下来我们对其进行测试.

测试步骤如下:

1.创建表

create table test (a int,b varchar2(20));

2.往这个表里INSERT 50000行数据

declare i int;  
　　begin for i in 1..50000 loop insert into test values(i,'adsfsafsa');  
　　end loop;  
end;

3.在A的栏位上创建一个INDEX

create index test_i on test(a);

4.编一个存储过程，进行大量的索引扫描操作

create or replace procedure test_k is
　　begin for i in (select /*+ INDEX(TEST TEST_I) */ * from test where a > 20000) loop null;  
　　end loop;  
end;

5.打开2000个会话来执行这个存储过程

var i number;  
　　begin  for j in 1..2000 loop dbms_job.submit(:i,'TEST_K;');  
　　commit;  
4　　 end loop;  
end;

6.查询等待事情发现latch: cache buffers chains

SELECT event,
  total_waits ,
  time_waited
FROM v$session_event
WHERE sid=
  (SELECT sid FROM v$mystat WHERE rownum=1
  )
ORDER BY 3 DESC;

EVENT                          TOTAL_WAITS TIME_WAITED
------------------------------ ----------- -----------
SQL*Net message from client            223      167283
resmgr:cpu quantum                      14        1022
db file sequential read                222          21
library cache lock                      47          12
control file sequential read         15400           7
latch: shared pool                       1           1
direct path write                       30           1
SQL*Net message to client              224           0
SQL*Net more data to client              1           0
SQL*Net break/reset to client           10           0
library cache: mutex X                   2           0
events in waitclass Other                6           0
log file sync                            4           0
enq: RO - fast object reuse              3           0
buffer busy waits                        3           0
latch: cache buffers chains              8           0
Disk file operations I/O                 8           0
latch: In memory undo latch              8           0

结论:就算以只读模式获取cache buffers chains LATCH并不能完全解决这个LATCH争用的问题,只是争用程度不那么严重了.