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Don Burleson Blog 






Cost Control: Inside the Oracle Optimizer

By Donald K. Burleson

Designing new applications for the Oracle Cost-Based Optimizer? Here's the latest information about how it works.

The goal of SQL tuning is to execute your SQL with the absolute minimum amount of I/O.  See my related SQL Optimization tips at the end this article.

Oracle's cost-based SQL optimizer (CBO) is an extremely sophisticated component of Oracle that governs the execution for every Oracle query. The CBO has evolved into one of the world's most sophisticated software components, and it has the challenging job of evaluating any SQL statement and generating the "best" execution plan for the statement.

Because the CBO determines the execution speed for every Oracle query, the Oracle professional must understand how the CBO is influenced by Oracle external issues, internal statistics, and data distribution.

In this first installment of a two-part article, we will cover the following CBO topics:

  • CBO parameters. We will start by reviewing the basic optimizer modes within the CBO and then           drill down and examine specific parameters that influence the behavior of the CBO.
  • CBO statistics. We will examine the importance of gathering proper CBO statistics with     dbms_stats and review techniques for ensuring that execution plans remain stable. We will also look         at techniques for migrating statistics between systems and examine how developers can optimize their
    SQL in a test environment and confidently migrate SQL into production without fear of changing execution plans.



This article has the following sections:

What's new in Oracle Database 10g?

With the advent of Oracle Database 10g we now see dramatic internal improvement to the cost-based SQL optimizer and easier mechanisms for automatic SQL optimization. The important SQL optimizer changes to Oracle Database 10g include the following exciting topics:

  • Rule-based optimizer de-support While the rules-based optimizer (RBO) exists inside Oracle Database 10g, Oracle highly recommends that those using rule-based optimization procrastinate no longer. Those sites that are still using the RBO can switch to first_rows optimizer_mode and adjust the parameter optimizer_index_cost_adj to a small number (< 25) to make the cost-based optimizer simulate the behavior of the RBO. Shops that do not want their Oracle Database 10g migration to change their execution plans can use Oracle's optimizer plan stability feature to preserve their rule-based execution plans prior to migrating.


  • User-Initiated Buffer Cache Flushing You can now flush the buffer cache manually between runs of test queries, which facilitates your diagnosing and testing of SQL run-time execution. For SQL unit testing, this ability to clear the data buffers ensures uniform SQL response time testing and removes the performance variability associated with RAM data caching.


  • SQLAccess Advisor The SQLAccess Advisor is an expert system inside the dbms_advisor package that identifies (and advises on resolution) of SQL execution performance problems. It analyzes SQL from the library cache and recommends which indexes or materialized views to create, drop, or retain.

Cost Basis

While we have gone into great detail on the optimizer, there is always more to learn as the optimizer becomes more powerful (and complex) with each new release. The main points of this article include general guidelines for adjusting the behavior of the optimizer:

  • Histograms provide detailed column information to the optimizer in rare cases when the value of an index column would change the optimal execution plan. Hence, histograms should be used only when justified.
  • Writing SQL that gets the correct data is not enough. Developers should be held responsible for tuning their SQL and should be trained in optimal SQL formatting and understand how to use explain plan and TKPROF.
  • The most common problems with SQL optimization are missing indexes (or non-selective indexes) and sub-optimal table join methods.
  • Oracle hints are used to change execution plans for a query, but should only be used as a last resort.
  • The v$sql_plan view shows the execution plan for all SQL in your library cache and you can query this view to get useful insights into SQL execution internals.

Special thanks to Oracle SQL Guru Andrew Holdsworth for his assistance.

Oracle 11g Notes

Even though Oracle has deprecated the rule-based optimizer, Oracle continues to use the rule hint in Oracle 11g, as shown by this Data Pump internal SQL:

 Module: Data Pump Worker
'TABLE_T', '7')), KU$.OBJ_NUM ,KU$.ANC_OBJ.NAME  . . .  

My related notes on Oracle SQL optimization:

Get the Complete
Oracle SQL Tuning Information 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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