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Determining the Default Optimizer_Mode

Oracle Tips by Burleson Consulting

As we have noted, there are some shortcomings in the CBO and the RBO, and you must make a decision about the appropriate optimizer_mode default for your database. If your database has many thousands of SQL statements to tune, choosing the best overall optimizer mode can save you hundreds of hours of SQL tuning effort. Every database is different, both in the types of queries and the structures of the data. However, there are some general rules that can be applied:

Prior to Oracle 8.1.6, queries that join three or more large tables will generally benefit from the rule-based optimizer or the first_rows hint.

  • Queries that access bitmapped or function-based indexes will benefit from the cost-based optimizer.

  • Queries that will benefit from hash joins

  • Queries that use STAR query hints need the cost-based optimizer.

  • Databases at Oracle8i and beyond may benefit from the cost-based optimizer default.

The choice of default optimizers depends on the version of Oracle. Oracle recommends that all Oracle7 databases use rule-based optimization, and by Oracle8i, the cost-based optimizer has improved to the point where it can be considered for a default optimizer mode. Given that any Oracle environment would benefit from both optimizers, there are several choices. The Remote DBA could make the cost-based optimizer the default and use rule hints when required, or they could make rule-based the default and use cost hints and statistics when desired.

Many Remote DBAs conduct a study where they bounce the init.ora optimizer_mode and then run the application for a day in each mode and collect statistics. From these overall comparisons, the proper default optimizer_mode becomes readily apparent. Here is the procedure:

Notify your end-users that you are changing important parameters and ask them to call you if they notice any significant changes in performance. Also tell the end-users that you can quickly put the system back into its original state.

  • Change the initialization file to change optimizer_mode.

  • Bounce the database

  • Use tools to measure end-to-end response time for the most important transactions.

Again, the first step in any Oracle SQL tuning endeavor is to set the appropriate default for the whole database. This may save hundreds of hours of manual tuning.

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

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