Saturday, June 8, 2013

Visual Explain explained

Visual Explain in Db2 is a great tool which shows you how the query optimizer actually processes your sql statements. It gives you every information that you need to improve your sql statements or gives you hints which indexes you should create to get a better performance. Here is an example of the output of visual explain:


Every little icon represents one process step in your query. Some of the icons are pretty clear. Everyone knows what a table scan or and index probe means. But what about the others? The online help of Visual explain is not really helpfull, because there are only very short descriptions of every icon. But fortunatly there is a detailed description with examples of every access method the query optimizer can choose to process your sql statements in the IBM i information center. If you prefer to read this informations on paper you can download a PDF from

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