Oracle诊断案例-Sql_trace之一

link:

http://www.eygle.com/case/sql_trace_1.htm

问题描述:

这是帮助一个公司的诊断案例.
应用是一个后台新闻发布系统.

症状是,通过连接访问新闻页是极其缓慢
通常需要十数秒才能返回.

这种性能是用户不能忍受的.

操作系统:SunOS 5.8
数据库版本:8.1.7

1.检查并跟踪数据库进程

诊断时是晚上,无用户访问
在前台点击相关页面,同时进行进程跟踪

查询v$session视图,获取进程信息

SQL> select sid,serial#,username from v$session; SID SERIAL# USERNAME ---------- ---------- ------------------------------ 1 1 2 1 3 1 4 1 5 1 6 1 7 284 IFLOW 11 214 IFLOW 12 164 SYS 16 1042 IFLOW 10 rows selected.

启用相关进程sql_trace

等候一段时间,关闭sql_trace

SQL> exec dbms_system.set_sql_trace_in_session(7,284,false) PL/SQL procedure successfully completed. SQL> exec dbms_system.set_sql_trace_in_session(11,214,false) PL/SQL procedure successfully completed. SQL> exec dbms_system.set_sql_trace_in_session(16,1042,false) PL/SQL procedure successfully completed.

2.检查trace文件

检查发现以下语句是可疑的

这里显然是根据articleId进行新闻读取的.
很可疑的是query读取有3892

这个内容引起了我的注意.
如果遇到过类似的问题，大家在这里就应该知道是怎么回事情了.
如果没有遇到过的朋友，可以在这里思考一下再往下看.

Misses in library cache during parse: 1 Optimizer goal: CHOOSE Parsing user id: 41 Rows Row Source Operation ------- --------------------------------------------------- 1 NESTED LOOPS 2 INDEX RANGE SCAN (object id 25062) 1 TABLE ACCESS BY INDEX ROWID CATEGORY 2 INDEX UNIQUE SCAN (object id 25057) ******************************************************************************** select auditstatus,categoryid from categoryarticleassign where articleId=20030700400138 and categoryId in ('63', '138','139','140','141','142','143','144','168','213','292','341','346', '347','348','349','350','351','352','353','354','355','356','357','358', '359','360','361','362','363','364','365','366','367','368','369','370', '371','372','383','460','461','462','463','621','622','626','629','631', '634','636','643','802','837','838','849','850','851','852','853','854', '858','859','860','861','862','863','-1') call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 1 4.91 4.91 0 2835 7 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 3 4.91 4.91 0 2835 7 1 Misses in library cache during parse: 1 Optimizer goal: CHOOSE Parsing user id: 41 Rows Row Source Operation ------- --------------------------------------------------- 1 'TABLE ACCESS FULL CATEGORYARTICLEASSIGN' 我们注意到，这里有一个全表扫描存在 ********************************************************************************

3.登陆数据库,检查相应表结构

我们注意到,IDX_ARTICLEID索引在以上查询中都没有被用到.

检查表结构:

SQL> desc categoryarticleassign Name Null? Type ----------------------------------------- -------- ---------------------------- CATEGORYID NOT NULL NUMBER ARTICLEID NOT NULL VARCHAR2(14) ASSIGNTYPE NOT NULL VARCHAR2(1) AUDITSTATUS NOT NULL NUMBER SORTID NOT NULL NUMBER UNPASS VARCHAR2(255)

问题发现:
因为ARTICLEID是个字符型数据,查询中给入的articleId= 20030700400141 是一个数字值
Oracle发生潜在的数据类型转换,从而导致了索引失效

SQL> select auditstatus,categoryid 2 from 3 categoryarticleassign where articleId=20030700400132; AUDITSTATUS CATEGORYID ----------- ---------- 9 94 0 383 0 695 Elapsed: 00:00:02.62 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=110 Card=2 Bytes=38) 1 0 TABLE ACCESS (FULL) OF 'CATEGORYARTICLEASSIGN' (Cost=110 Card=2 Bytes=38)

4.解决方法

简单的在参数两侧各增加一个',既可解决这个问题.

对于类似的查询,我们发现Query模式读取降低为2
几乎不需要花费CPU时间了

******************************************************************************** select unpass from categoryarticleassign where articleid='20030320000682' and categoryid='113' call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 1 0.00 0.00 0 2 0 0 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 3 0.00 0.00 0 2 0 0 Misses in library cache during parse: 1 Optimizer goal: CHOOSE Parsing user id: 20 Rows Row Source Operation ------- --------------------------------------------------- 0 TABLE ACCESS BY INDEX ROWID CATEGORYARTICLEASSIGN 1 INDEX RANGE SCAN (object id 3080) ********************************************************************************

至此,这个问题得到了完满的解决.