【IT168 技术文章】
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~1\IBM\SQLLIB\" with DB2 Copy Name
"DB2COPY1".
<---------------创建一个数据库
D:\TEMP\db2service.perf1>db2 create db sampel2DB20000I The CREATE DATABASE command completed successfully.
D:\TEMP\db2service.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:\TEMP\db2service.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:\TEMP\db2service.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 Files\IBM\SQLLIB\MISC
D:\Program Files\IBM\SQLLIB\MISC>db2 -tvf EXPLAIN.DDL
.....
D:\Program Files\IBM\SQLLIB\MISC>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
