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Arthur C. Clarke once wrote that “any sufficiently advanced technology is indistinguishable from magic. ” I believe that observation to be entirely accurate. Someone else later observed that “any technologist with sufficient knowledge is indistinguishable from a magician. ” With that in mind, what you have in your hands right now is a book on magic. However, for as long as I’ve known the author of this book, Jonathan Lewis (some 11 years according to my research on Google going back over the newsgroup archives), he has never been content to accept “magic. ” He wants to know why something happens the way it does. So, fundamentally, his book is all about understanding: understand v. understood, (-std) understanding, understands 1. To perceive and comprehend the nature and significance of 2. To know thoroughly by close contact or long experience with More precisely, it is all about understanding the Oracle cost based optimizer (CBO), how it works, and why it does what it does. Jonathan conveys to us his understanding of the Oracle CBO through practice and example, and with this understanding, with this knowledge, new options and solutions become available. Put simply, the Oracle CBO is a mathematical model; you feed it inputs (queries, statistics), and it produces outputs (query plans). In order to use the CBO successfully, it is critical that you understand what these inputs are and how the CBO uses them.