Download Optimizer - Intelligent Data Systems Laboratory

DB Tuning : Chapter 10. Optimizer
이상근
Intelligent Database Systems Lab
School of Computer Science & Engineering
Seoul National University, Seoul, Korea
Center for E-Business Technology
Seoul National University
Seoul, Korea
Optimizer ?
 Optimizer is like brain of database system
 Optimizer is one of Oracle DBMS processes. It finds the best(?)
execution plan of input SQL query
Optimizer
Query Rewrite
Query Optimizing
Optimizer Select
Rule based Optimizer
SQL Parsing
SELECT * FROM EMP
What we do when we tune :
Helping optimizer to make
better decision
Important factors:
Index, Partition, Parallel
Processing
CLIENT
Cost based Optimizer
Query Execution plan
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Important Tuning Points: SQL Parsing
 Cursor Shared

In large data warehouse system, it may not be important.
However, OLTP system like web system. 1 second different can
be very significant. We’d better bind variable to share cursor for
the same SQL queries that have different variables
 Shared Pool Size

Should be enough
 Eliminating useless columns and tables

Trivial
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More about optimizer
 Rule-based Optimizer

It has several priority rules in it. Following the rules, it finds the
best execution plan

Rank 1 : Table Access using ROWID

Rank 2 : Table Access using Unique Index

Rank 3 : Table Access using Index

Rank 4 : Full Table Scan

Pros
–
Developers can choose the execution plan as they want
–
If you know how it works, you can use it as you want (of course not
easy)
–
Latest version of Oracle Rule-based optimizer use statistics too!
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More about optimizer

Cost-based Optimizer

It uses statistics to find the best execution plan

Analyzing needs to lock tables -> Overhead

Not as good as we hope
–

Can’t be better than the execution plan that people made
Type:
–
First Row

–
All Rows


Select * from EMP;
Select count(*) from EMP;
The author doesn’t recommend cost-based optimizer

We can’t easily guess what the execution plan will be

Not easy to modify execution plan
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Execution Plan
 The results made by optimizer
 It’s a plan how a SQL will be executed
 How to check out execution plan


Many ways to do this
–
Explain
–
Autotrace
–
TKPROF Trace
–
10046 Trace
Read from page.233~
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Semantic Tech & Context - 6