Topic: DB2 for z/OS
There is always a lot of talk about the DB2 optimizer and what is the best access path for a particular query. While this is of course important it is also important to know that the optimizer is only part of the story. When a query is submitted one of the first thing that happens is that it goes through query transformation. In this part of the process DB2 interprets the submitted SQL, and rewrites it. Views and UDFs are merged with the query, subqueries possibly transformed to joins, table expressions merged, and much more. If you are a serious query tuner you need to understand what is happening during query transformation and what you can do to influence it.
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