【原】Cache Buffer Chain 第四篇

作者：david_zhang@sh 【转载时请以超链接形式标明文章】

链接：http://www.cnblogs.com/david-zhang-index/p/3873357.html

【测试1】低效的SQL引起的catch buffers chains 锁存器争用

创建测试表

create table cbc_test(id number,name char(100));
insert into cbc_test(id,name) select rownum,object_name from dba_objects;

创建索引

create index cbc_test_idx on cbc_test(id);

创建扫描表的procedure

create or replace procedure cbc_do_select
2 is
3 begin
4 for x in (select /*+ INDEX(cbc_test cbc_test_idx)*/ * from cbc_test where id >= 0) loop
5 null;
6 end loop;
7 end;
8 /

同时打开2000个会话执行读取工作

var job_no number;
begin
for idx in 1..2000 loop
dbms_job.submit(:job_no,'cbc_do_select;');
commit;
end loop;
end;
/

查看整个系统当前等待

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

 结果如下：

EVENT                          TOTAL_WAITS TIME_WAITED
------------------------------ ----------- -----------
SQL*Net message from client             44       46352
library cache lock                     202         253
buffer busy waits                       56         113
latch: In memory undo latch             43          86
latch: cache buffers chains             33          55
control file sequential read         14154          22
db file sequential read                 82          18
events in waitclass Other                6          11
library cache: mutex X                   8          10
latch: shared pool                       4           5
latch: row cache objects                 1           2
SQL*Net message to client               45           0
Disk file operations I/O                 4           0
SQL*Net break/reset to client            1           0
log file sync                            1           0

根据cache buffers chains事件，找出对应的latch地址

select addr, gets, misses, sleeps
  from v$latch_children
 where name = 'cache buffers chains'
   and misses > 100000
 order by 3 desc;

结果：

ADDR                   GETS     MISSES     SLEEPS
---------------- ---------- ---------- ----------
0000000124ECCBB0   10485987    5260833          0
0000000124E2B5A8   20671341    9806756      27942

根据ADDR找出关联的文件号和块号

SQL> select FILE#,DBABLK,CLASS,STATE from x$bh where HLADDR='0000000124E2B5A8';

     FILE#     DBABLK      CLASS      STATE
---------- ---------- ---------- ----------
         4    3585766          1          1
         3       9411         20          1
         4    3651302          1          1
         4    3581428          1          1
         6      71918          1          1
         6      71918          1          3
         6      71918          1          3
         6      71918          1          3
         6      71918          1          3
         6      71918          1          3
         3       5073         38          0

根据文件号和块号，找出对象名称，输入6和71918

select owner, segment_name
  from dba_extents
 where file_id = &p1
   and &p2 between block_id and block_id + blocks - 1;

找出的对象名称

OWNER                          SEGMENT_NAME
------------------------------ ------------------------------
SCOTT                           EMP_FP_IDX1

待续。。。