It is a capital mistake to theorize before one has the data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.

Sir Arthur Conan Doyle (1859 1930)

If you ever wanted to learn to play the piano, there are many different approaches that people have tried over the past three centuries. The traditional recommended way is to sign up with a professional teacher and take lessons one-on-one. Others have tried a variety of methods of self-instruction, such as using books like Piano for Dummies, watching videos, or just getting some sheet music and start playing. Regardless of which method you choose, there is another variable, how to proceed to learn the piece you want to play: hands separate and then together, hands together from the beginning, learning in sections, etc. Therefore, to get to the final goal, there are literally dozens and possibly hundreds of paths one can take to get the final correct result. The quality of the result may vary, but you can get there many different ways. For database queries there is also a multipath possibility to get the same correct result, and it is important to be able to analyze the different paths for the quality of the result, in other words, the performance of the system to get you the correct result and choose the best path to get you there.

This chapter focuses on the basic concepts of query optimization needed to understand the interactions between physical database...