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Introduction
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To understand how SQL optimizers decide what table and index scans should be performed to process SQL statements as ef ciently as possible To be able to quantify the work being done during these scans to enable satisfactory index design Type and background of audience for whom the book is written Initial thoughts on the major reasons for inadequate indexing Systematic index design.
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ANOTHER BOOK ABOUT SQL PERFORMANCE!
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Relational databases have been around now for over 20 years, and that s precisely how long performance problems have been around too and yet here is another book on the subject. It s true that this book focuses on the index design aspects of performance; however, some of the other books consider this area to a greater or lesser extent. But then a lot of these books have been around for over 20 years, and the problems still keep on coming. So perhaps there is a need for a book that goes beyond the usual boundaries and starts to think about why so many people are still having so many problems. It s certainly true that the world of relational database systems is a very complex one it has to be if one re ects on what really has to be done to satisfy SQL statements. The irony is that the SQL is so beautifully simple to write; the concept of tables and rows and columns is so easy to understand. Yet we could be searching for huge amounts of information from vast sources of data held all over the world and we don t even need to know where it is or how it can be found. Neither do we have to worry about how long it s going to take or how much it s going to cost. It all seems like magic. Maybe that s part of the problem it s too easy; but then of course, it should be so easy. We still recognize that problems will arise and huge problems at that. Stories abound of simple queries that might have been expected to take a fraction of a second appear to be quite happy to take several minutes or even longer. But then, we have all these books, and they tell us how to code the query
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Relational Database Index Design and the Optimizers, by Tapio Lahdenm ki and Michael Leach a Copyright 2005 John Wiley & Sons, Inc.
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1 Introduction
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properly and how to organize the table and what rules to follow to put the right columns into the index and often it works. But we still seem to continue to have performance problems, despite the fact that many of these books are really very good, and their authors really know what they are talking about. Of particular interest to us in this book is the part of the relational system (called the SQL optimizer) that decides how to nd all the information required in the most ef cient way it can. In an ideal world, we wouldn t even need to know it exists, and indeed most people are quite happy to leave it that way! Having made this decision, the optimizer directs scans of indexes and tables to nd our data. In order to understand what s going through the optimizer s mind, we will also need to appreciate what is involved in these scans. So what we want to do in this book is rst to try to put ourselves in the optimizer s place; how it decides what table and index scans should be performed to process SQL statements as ef ciently as possible. Perhaps if we understand why it might have problems, we could do things differently; not by simply following a myriad of incredibly complex rules that, even if we can understand them might or might not apply, but by understanding what it is trying to do. A major concern that one might reasonably be expected to have on hearing this is that it would appear to be too complex or even out of the question. But it is quite surprising how little we really need to understand ; what there is, though, is incredibly important. Likewise, perhaps the rst, and arguably the most important, difference this book has from other books in its eld is that we will not be providing a massive list of rules and syntax to use for coding SQL and designing tables or even indexes. This is not a reference book to show exactly which SQL WHERE clause should be used, or what syntax should be employed, for every conceivable situation. If we try to follow a long list of complicated, ambiguous, and possibly even incomplete instructions, we will be following all the others who have already trod the same path. If on the other hand we understand the impact of what we are asking the relational system to undertake, and how we can in uence that impact, we will be able to understand, avoid, minimize, and control the problems being encountered. A second objective of this book is to show how we can use this knowledge to quantify the work being performed. Only in this way can we truly judge the success of our index design; we need to be able to use actual gures to show what the optimizer would think, how long the scans would take, and what modi cations would be required to provide satisfactory performance. But most importantly, we have to be able to do this quickly and easily; this in turn means that it is vital to focus on a few major issues, not on the relatively unimportant detail under which many people drown. This is key to focus on a very few, crucially important issues and to be able to say how long it would take or how much it would cost. We have also one further advantage to offer, which again arises as a result of focusing on what really matters. For those who may be working with more than one relational product (even from the same vendor), instead of needing to read
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