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 iflow10 rows selected.
启用相关进程sql_trace
sql> exec dbms_system.set_sql_trace_in_session(7,284,true)pl/sql procedure successfully completed.sql> exec dbms_system.set_sql_trace_in_session(11,214,true)pl/sql procedure successfully completed.sql> exec dbms_system.set_sql_trace_in_session(16,1042,true)pl/sql procedure successfully completed.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 iflow10 rows selected.
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
********************************************************************************select auditstatus,categoryid,auditlevel from categoryarticleassign a,category b where b.id=a.categoryid and articleid= 20030700400141 and auditstatus>0call count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------parse 1 0.00 0.00 0 0 0 0execute 1 0.00 0.00 0 0 0 0fetch 1 0.81 0.81 0 3892 0 1------- ------ -------- ---------- ---------- ---------- ---------- ----------total 3 0.81 0.81 0 3892 0 1********************************************************************************
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: 1optimizer goal: chooseparsing 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 0execute 1 0.00 0.00 0 0 0 0fetch 1 4.91 4.91 0 2835 7 1------- ------ -------- ---------- ---------- ---------- ---------- ----------total 3 4.91 4.91 0 2835 7 1misses in library cache during parse: 1optimizer goal: chooseparsing 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
sql> select index_name,table_name,column_name from user_ind_columns 2 where table_name=upper('categoryarticleassign');index_name table_name column_name ------------------------------ ------------------------------ -------------------- idx_articleid categoryarticleassign articleidind_articleid_categ categoryarticleassign articleid ind_articleid_categ categoryarticleassign categoryid idx_sortid categoryarticleassign sortid pk_categoryarticleassign categoryarticleassign articleid pk_categoryarticleassign categoryarticleassign categoryid pk_categoryarticleassign categoryarticleassign assigntype idx_cat_article categoryarticleassign auditstatus idx_cat_article categoryarticleassign articleid idx_cat_article categoryarticleassign categoryid idx_cat_article categoryarticleassign assigntype 11 rows selected.
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: 00:00:02.62execution 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 0execute 1 0.00 0.00 0 0 0 0fetch 1 0.00 0.00 0 2 0 0------- ------ -------- ---------- ---------- ---------- ---------- ----------total 3 0.00 0.00 0 2 0 0misses in library cache during parse: 1optimizer goal: chooseparsing 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.
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