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OCP Instructors Guide for Oracle DBA Certification

Chapter 7 - Tuning and Performance

Optimizer Dynamic Sampling

The cost based optimizer uses statistics generated by the dbms_stats procedure or the ANALYZE statement as the situational data when creating its expert opinion on which access path to the data is most optimal. These statistics are stored in the data dictionary and describe the objects space characteristics, data uniqueness and data distribution. 

The cost-based optimizer is only as good as the statistics it uses as input.  Statistics collections should be run on a regular basis to ensure that the statistics are current (representative of the data being accessed).  The optimizer is then able to create a highly accurate access path that is based on the least cost.  If statistics are not available, the optimizer uses a simple algorithm to calculate the statistics, which often leads to “less than optimal” access paths.  In other words, Oracle guesses and it is usually not a very good guesser.

Oracle 9i Release 2 introduces optimizer dynamic sampling to overcome the lack of accurate statistics on the objects being accessed.  The optimizer is now able to take a sampling of the data during access path optimization.  Administrators are able to activate dynamic sampling and control the size of the dynamic sample taken by using the optimizer_dynamic_sampling dynamic initialization parameter as a throttle.

The above text is an excerpt from:

OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques

ISBN 0-9744355-3-8

by Christopher T. Foot

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