DB Tuning : Chapter 10. Optimizer Center for E-Business Technology Seoul National University Seoul, Korea 이상근 Intelligent Database Systems Lab School of Computer Science & Engineering Seoul National University, Seoul, Korea

Copyright  2008 by CEBT 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 Semantic Tech & Context - 2 Query Rewrite Query Optimizing SQL Parsing SELECT * FROM EMP CLIENT Rule based OptimizerCost based Optimizer Query Execution plan Optimizer Select Optimizer What we do when we tune : Helping optimizer to make better decision Important factors: Index, Partition, Parallel Processing

Copyright  2008 by CEBT 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 Semantic Tech & Context - 3

Copyright  2008 by CEBT 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! Semantic Tech & Context - 4

Copyright  2008 by CEBT 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 Select * from EMP; – All Rows 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 Semantic Tech & Context - 5

Copyright  2008 by CEBT 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 – Trace Read from page.233~ Semantic Tech & Context - 6