Articles & Content


Virtual Reality: Virtual Indexes That Aren't and Real Cardinality on Explains - D08

Topic: 2012 NA

Subtopic: DB2 for LUW

Have you ever wanted to try one or more new indexes to see whether the optimizer would use them effectively? But, you don't always have the resources or the time to create the index. Or, perhaps you are
dealing with a huge production table that you can't just go creating "test" indexes in. In this session you will learn how to create virtual indexes and run the explain to see how the index is used.

Estimated cardinality is one of the most important statistics on an explain to look at when trying to figure out why the DB2 optimizer made the choices that it did. But, what if the estimates that DB2 has made are inaccurate? Come learn about "section_actuals" and how to combine the execution of a query with the explain in order to see the actual cardinality for each step of an explain. Large mismatches between estimated and actual cardinality can often provide a major clue when tuning a difficult query.

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