CHAPTER 9Query Transformation

Nine chapters into the book, and I still haven't got as far as a two-table join. And I'm going to avoid joins for just one more chapter while I discuss features such as subqueries, view merging, unnesting, and the star transformation.

The reason for examining some of the more subtle options before looking at joins is that the optimizer tries to restructure the SQL you write, generally turning it into a simpler form consisting of just a straight join between tables, before optimizing it. This means the first step in understanding how the optimizer evaluates an execution plan requires you to work out thestructure of the SQL that is actually being optimized. Since this chapter is about transformation mechanisms, there ...

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