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
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$session视图,获取进程信息
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SQL> select sid,serial#,username from v$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.
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启用相关进程sql_trace
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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 IFLOW
10 rows selected.
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等候一段时间,关闭sql_trace
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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.
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2.检查trace文件
检查发现以下语句是可疑的
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********************************************************************************
select auditstatus,categoryid,auditlevel
from
categoryarticleassign a,category b where b.id=a.categoryid and articleId=
20030700400141 and auditstatus>0
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 0.81 0.81 0 3892 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 3 0.81 0.81 0 3892 0 1
********************************************************************************
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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
********************************************************************************
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3.登陆数据库,检查相应表结构
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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:
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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)
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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
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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)
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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)
********************************************************************************
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至此,这个问题得到了完满的解决.