More on Query Optimization for Oracle Databases - Saving the Optimization
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There may be times when you want to prevent the optimizer from calculating a new plan whenever a SQL statement is submitted. For example, you might do this if you’ve finally reached a point at which you feel the SQL is running optimally, and you don’t want the plan to change regardless of future changes to the optimizer or the database.
Starting with Oracle8i, you could create a stored outline that stored the attributes used by the optimizer to create an execution plan. Once you had a stored outline, the optimizer simply used the stored attributes to create an execution plan. As of Oracle9i, you could also edit the hints that were in the stored outline.
With the release of Oracle Database 11g, Oracle suggests that you move your stored outlines to SQL plan baselines. Now, in addition to manually loading plans, Oracle can be set to automatically capture plan histories into these SQL plan baselines. Included in this gathered history is the SQL text, outline, bind variables, and compilation environment. When a SQL statement is compiled, Oracle will first use the cost-based optimizer to generate a plan and will evaluate any matching SQL plan baselines for relative cost, choosing the plan with the lowest cost.
Comparing Optimizations
Oracle makes changes to the optimizer in every release. These changes are meant to improve the overall quality of the decisions the optimizer makes, but a generally improved optimizer could still create an execution plan for any particular SQL statement that could result in a decrease in performance.
The SQL*Analyzer tool is designed to give you the ability to recognize potential problems caused by optimizer upgrades. This tool compares the execution plans for the SQL statements in your application, flagging the ones in which the plans differ.
Once these statements are identified, SQL*Analyzer executes the SQL in each environment and provides feedback on the performance and resource utilization for each. Although SQL*Analyzer cannot avoid potential problems brought on by optimizer upgrades, the tool can definitely simplify an otherwise complex testing task.
Oracle Database 11g also includes a feature called Database Replay. This feature captures workloads from production systems and allows them to be run on test systems. With this capability, you can test actual production scenarios against new configurations or versions of the database, and Database Replay will spot areas of potential performance problems on the changed platform.
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 This article is excerpted from chapter four of the book Oracle Essentials, Fourth Edition Oracle Database 11g, written by Rick Greenwald, Robert Stackowiak, and Jonathan Stern (O'Reilly, 2007; ISBN: 0596514549). Check it out today at your favorite bookstore. Buy this book now.
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