很多开发和数据库管理人员都在为优化器问题烦恼不已。尽管很多时候优化器问题都是可以通过常规手段解决的，但是在某些特殊情况下，或者紧急情况（没有时间完整地分析问题）下，用户可以使用profile暂时强制优化器使用某些特定的操作。。。
下面是一个step by step的例子，简单地说明了怎样强制优化器使用table scan
DB21085I Instance "DB2" uses "32" bits and DB2 code release "SQL09010" with
level identifier "02010107".
Informational tokens are "DB2 v9.1.0.356", "s060629", "NT32", and Fix Pack "0".
Product is installed at "D:PROGRA~1IBMSQLLIB" with DB2 Copy Name
"DB2COPY1".

<---------------创建一个数据库
D:TEMPdb2service.perf1>db2 create db sampel2DB20000I The CREATE DATABASE command completed successfully.

D:TEMPdb2service.perf1>db2 connect to sampel2

Database Connection Information

Database server = DB2/NT 9.1.0
SQL authorization ID = TAOEWANG
Local database alias = SAMPEL2

<----------创建优化器系统表
D:TEMPdb2service.perf1>db2 "create table systools.opt_profile (schema VARCHAR(128) not null, name varchar(128) not null, profile blob (2M) not null, primary key (schema, name))"
DB20000I The SQL command completed successfully.

D:TEMPdb2service.perf1>cd ..

<----------创建用户表
D:TEMP>db2 "create table mytable (name varchar(128), id integer, salary float,phone varchar(20))"
DB20000I The SQL command completed successfully.

<-----------插入一些数据
D:TEMP>db2 "insert into mytable values ('tao wang', 12345, 100, '123-456')"
DB20000I The SQL command completed successfully.

D:TEMP>db2 "insert into mytable values ('diablo2', 12346, 101, '123-457')"
DB20000I The SQL command completed successfully.

D:TEMP>db2 "insert into mytable values ('whiterain', 123, 102, '123-458')"
DB20000I The SQL command completed successfully.

D:TEMP>db2 "insert into mytable values ('ganquan', 1255, 104, '123-459')"
DB20000I The SQL command completed successfully.

<---------别忘了runstats
D:TEMP>db2 "runstats on table taoewang.mytable"
DB20000I The RUNSTATS command completed successfully.

D:TEMP>db2 "runstats on table taoewang.mytable for indexes all"
DB20000I The RUNSTATS command completed successfully.


<-----------试试看
D:TEMP>db2 "SELECT * FROM TAOEWANG.MYTABLE WHERE ID < 1000"

NAME
ID SALARY
PHONE
-------------------------------------------------------------------------------------------------------------------------------- ----------- ------------------------ --------------------
whiterain
123 +1.02000000000000E+002 123-458

1 record(s) selected.

<--------创建explain表
D:TEMP>cd D:Program FilesIBMSQLLIBMISC

D:Program FilesIBMSQLLIBMISC>db2 -tvf EXPLAIN.DDL
.....

D:Program FilesIBMSQLLIBMISC>cd D:temp

<----------看一看现在的访问计划
D:TEMP>db2 set current explain mode explain
DB20000I The SQL command completed successfully.

D:TEMP>db2 "SELECT * FROM TAOEWANG.MYTABLE WHERE ID < 1000"
SQL0217W The statement was not executed as only Explain information requests
are being processed. SQLSTATE=01604

D:TEMP>db2 set current explain mode no
DB20000I The SQL command completed successfully.

D:TEMP>db2exfmt -d sampel2 -g TIC -w -1 -n % -s % -# 0 -o output.txt
DB2 Universal Database Version 9.1, 5622-044 (c) Copyright IBM Corp. 1991, 2006
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool

Connecting to the Database.
Connect to Database Successful.
Binding package - Bind was Successful
Output is in output.txt.
Executing Connect Reset -- Connect Reset was Successful.

D:TEMP>uedit32 output.txt

<-------------现在优化器用了index scan
Original Statement:
------------------
SELECT *
FROM TAOEWANG.MYTABLE
WHERE ID < 1000


Optimized Statement:
-------------------
SELECT Q1.NAME AS "NAME", Q1.ID AS "ID", Q1.SALARY AS "SALARY", Q1.PHONE AS
"PHONE"
FROM TAOEWANG.MYTABLE AS Q1
WHERE (Q1.ID < 1000)

Access Plan:
-----------
Total Cost: 7.56853
Query Degree: 1

Rows
RETURN
( 1)
Cost
I/O
|
1
FETCH
( 2)
7.56853
1
/----+---
1 4
IXSCAN TABLE: TAOEWANG
( 3) MYTABLE
0.00630865
0
|
4
INDEX: TAOEWANG
IX1

<---------创建一个xml文件，叫做a1.xml
a1.xml:



TAOEWANG">








<----------创建一个del文件,叫insert.del
insert .del:
"TAOEWANG", "PROF1", "a1.xml"

<--------用import把xml插入profile系统表
D:TEMP>db2 import from insert.del of del modified by lobsinfile insert into systools.opt_profile
SQL3109N The utility is beginning to load data from file "insert.del".

SQL3110N The utility has completed processing. "1" rows were read from the
input file.

SQL3221W ...Begin COMMIT WORK. Input Record Count = "1".

SQL3222W ...COMMIT of any database changes was successful.

SQL3149N "1" rows were processed from the input file. "1" rows weresuccessfully inserted into the table. "0" rows were rejected.


Number of rows read = 1
Number of rows skipped = 0
Number of rows inserted = 1
Number of rows updated = 0
Number of rows rejected = 0
Number of rows committed = 1

<--------设置db2set env variable打开profile
D:TEMP>db2set DB2_OPTPROFILE=YES

<--------重起实例让db2set生效
D:TEMP>db2stop force
12/27/2007 08:54:45 0 0 SQL1064N DB2STOP processing was successful.
SQL1064N DB2STOP processing was successful.

D:TEMP>db2start
12/27/2007 08:54:48 0 0 SQL1063N DB2START processing was successful.
SQL1063N DB2START processing was successful.

D:TEMP>db2 connect to SAMPEL2

Database Connection Information

Database server = DB2/NT 9.1.0
SQL authorization ID = TAOEWANG
Local database alias = SAMPEL2


D:TEMP>db2 set current explain mode explain
DB20000I The SQL command completed successfully.

D:TEMP>db2 set current schema taoewang
DB20000I The SQL command completed successfully.

<---------设置需要使用的profile
D:TEMP>db2 set current optimization profile='PROF1'
DB20000I The SQL command completed successfully.

D:TEMP>db2 "SELECT * FROM TAOEWANG.MYTABLE WHERE ID < 1000"
SQL0217W The statement was not executed as only Explain information requests
are being processed. SQLSTATE=01604

D:TEMP>db2 set current explain mode no
DB20000I The SQL command completed successfully.

D:TEMP>db2exfmt -d sampel2 -g TIC -w -1 -n % -s % -# 0 -o output2.txt
DB2 Universal Database Version 9.1, 5622-044 (c) Copyright IBM Corp. 1991, 2006
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool

Connecting to the Database.
Connect to Database Successful.
Output is in output2.txt.
Executing Connect Reset -- Connect Reset was Successful.

<------现在用了tablescan了
D:TEMP>uedit32 output2.txt
Profile Information:
--------------------
OPT_PROF: (Optimization Profile Name)
TAOEWANG.PROF1
STMTPROF: (Statement Profile Name)
Use Table Scan instead of Index Scan

Original Statement:
------------------
SELECT *
FROM TAOEWANG.MYTABLE
WHERE ID < 1000


Optimized Statement:
-------------------
SELECT Q1.NAME AS "NAME", Q1.ID AS "ID", Q1.SALARY AS "SALARY", Q1.PHONE AS
"PHONE"
FROM TAOEWANG.MYTABLE AS Q1
WHERE (Q1.ID < 1000)

Access Plan:
-----------
Total Cost: 7.56912
Query Degree: 1

Rows
RETURN
( 1)
Cost
I/O
|
1
TBSCAN
( 2)
7.56912
1
|
4
TABLE: TAOEWANG
MYTABLE




Extended Diagnostic Information:
--------------------------------

No extended Diagnostic Information for this statment.

如果用户希望在应用程序里面使用profile,可以使用下面的几种方法之一

对于cli应用程序中使用EXEC SQL SET CURRENT OPTIMIZATION PROFILE = 'xxxxxxxxx';
也可以在db2cli.ini中指定CURRENTOPTIMIZATIONPROFILE='"SCHEMA"."PROFILE"'
对于stored procedure在bind的时候指定OPTPROFILE

再次强调,profile不是万能药,只是止痛药~~~只有在万不得已的情况下才应该使用profile暂时指定用户需要的操作.一般来说,对于优化器问题用户应该尽量找到root cause,而不是简单地指定一个profile了事~~~