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Introduction
When I wrote Practical Oracle 8i, there was a three-week lag between publication and the first e-mail
asking me when I was going to produce a 9i version of the book—thanks to Larry Ellison’s timing of the
launch of 9i. That question has been repeated many times (with changes in version number) over the
last 12 years. This book is about as close as I’m going to come to writing a second edition of the book—
but it only covers the first chapter (and a tiny bit of the second and third) of the original.
There were two things that encouraged me to start writing again. First, was the number of times I
saw questions of the form: How does Oracle do XXX? Second, was the realization that it’s hard to find
answers to such questions that are both adequate and readable. Generally, you need only hunt through
the manuals and you will find answers to many of the commonly-asked questions; and if you search the
internet, you will find many articles about little features of how Oracle works. What you won’t find is a
cohesive narrative that put all the right bits together in the right order to give you a picture of how the
whole thing works and why it has to work the way it does. This book is an attempt to do just that. I want
to tell you the story of how Oracle works. I want to give you a narrative, not just a collection of bits and
pieces.
Targets
Since this book is only a couple of hundred pages and the 11g manuals extend to tens of thousands of
pages, it seems unlikely that I could possibly be describing “the whole thing,” so let me qualify the claim.
The book is about the core mechanics of the central database engine—the bit that drives everything else;
essentially it boils down to undo, redo, data caching, and shared SQL. Even then I’ve had to be ruthless
in eliminating lots of detail and interesting special cases that would make the book too long, turgid, and
unreadable. Consider, for example, the simple question: How does Oracle do a logical I/O?, then take a
look at structure x$kcbsw, which is a list of all the functions that Oracle might call to visit a block. You will
find (for 11.2.0.2) that there are 1,164 different functions for doing a logical I/O—do you really want a
detailed breakdown of all the options, or would a generic description of the common requirements be
sufficient?
The problem of detail repeats itself at a different level—how much rocket science do you want to
know; and how much benefit would anyone get from the book be if I did spend all my time writing about
some of the incredibly intricate detail. Again, there’s a necessary compromise to reach between
completeness, accuracy, and basic readability. I think the image I’ve followed is one that I first saw
expressed by Andrew Holdsworth of Oracle’s Real-World Performance Group at Oracle OpenWorld in
2006. In a presentation about the optimizer and how to collect statistics, he talked about the 90/9/1
methodology, as follows:
• 90 percent of the time the default sample works
• 9 percent of the time a larger sample works
• 1 percent of the time the sample size is irrelevant
