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OCP Instructors Guide for Oracle DBA Certification

Chapter 8 - Miscellaneous Oracle Information

The following sections provide information on a wide range of topics. Information provided includes general hints and tips to simplify administrative tasks and information that, although important, didnít fit into any other section.

The Foot Rule of Thumb

The Foot Rule of Thumb is that there are no rules of thumb! Donít listen to industry pundits about their rules. Make your own rules. How do you do that? Test! Experiment! Learn! Afraid that performance will suffer if you place 6 columns in an index to obtain index-only access? Build the index, monitor on-line transaction performance and find out. The index 95/5 rule (donít build indexes on queries that retrieve more than 5% of the rows) is a high-level recommendation. It doesnít ring true in all cases. Every environment is different and there are no two workload mixes that are the same. I have seen queries return much higher percentages of rows through indexes and the queries performed flawlessly. Remember rules of thumb are just that, general recommendations.

Try different column combinations, multiple index usage, clustering changes, etc. When prototyping complex joins between several tables, build a set of indexes that favors one access path and test the queries in question. Drop the first set of indexes and create indexes that favor another access path (such as two different tables being joined first or a different join method being used) and test again. Build the different permutations of indexes that influence the optimizer to join a different set of tables first or allow different join types to be performed. Test each different access path and keep a performance history of each change that is made. The above hint may seem to be a time consuming process, but until you learn access paths this is the best approach to take. I have spent hundreds of hours working with access paths and if I could impart one sentence of wisdom, it would be the following one: What path works well in one situation may not be the best choice for another situation that almost exactly duplicates the first!

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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