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The SQL Optimizer

Oracle Tips by Burleson Consulting

In a perfect world, the SQL optimizer should be able to generate the most efficient method to service the data request. If the SQL optimizer has all of the information about all of the tables involved in the query (e.g., the number of rows, the distribution of values within a column), the optimizer should always choose the best execution plan for accessing the data.

In the real world, the SQL optimizer requires human assistance. It is the responsibility of the Oracle Remote DBA to ensure that all of the object statistics are available to the SQL optimizer, and the overwhelming complexity of SQL optimization makes it very difficult for any vendor to deliver an optimizer that always chooses the best access path to the data. Hence, we need a book such as this to understand how to help the optimizer choose the most efficient path to the data.

The Goals of SQL Tuning

SQL tuning is the most time-consuming and challenging of all Oracle tuning activities. Unlike SGA tuning, which is performed only once, SQL tuning is an ongoing process. Each SQL statement must be individually tuned, and the tuning must be made permanent by using hints or with the new Oracle8i optimizer plan stability feature.  For details on using optimizer plan stability, please see Chapter 13.

Also, SQL tuning has the most immediate positive impact on Oracle performance. When the Oracle optimizer chooses a less-than-optimal execution plan, human tuning can often triple the speed of the query. For queries that are executed thousands of time per day, this can result in a dramatic improvement of database throughput and extend the life of the hardware on the database server. The following list summarizes the goals of SQL tuning:

  • To ensure that all SQL has been “approved” for performance It is important to note that the goals of the programmers who write the SQL are very different from the goals of the person charged with tuning the SQL. The programmers are concerned with getting a query with the correct result  as quickly as possible, and it is not uncommon to find SQL that is less than optimal from a performance perspective.
  • To create Oracle statistics and indexes that will benefit all SQL The Oracle SQL optimizer does not exist in a vacuum, and it is the goal  of the Oracle Remote DBA to create appropriate indexes and object analysis techniques to ensure that all SQL will perform in an optimal fashion.
  • To lock down the execution plan for all SQL Adding a new  index can sometimes result in a change to dozens of SQL statements, and a primary goal of SQL tuning is to make the tuning changes permanent. For any SQL statement, there is only one optimal execution plan, and once that is identified, it is the goal of the Oracle Remote DBA to make the execution plan permanent by adding hints to the query or by using optimizer plan stability.

We will be discussing these goals as a central theme throughout the body of this book.


This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.


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