Optimizer Transformations

Abstract

We have seen throughout this book that there are many ways to transform one SQL query (or DML subquery) into another without in any way changing the query's meaning. For all but the most trivial of queries the CBO will consider a number of such transformations, sometimes estimating costing, to determine the best SQL statement on which to perform the final state optimization that we just covered in Chapter 12. This chapter is dedicated to the topic of optimizer transformations; you will see that there are a bewildering number of such transformations. Although not all optimizer transformations are included in this chapter, the vast majority that you will encounter in your day-to-day life are, as well as some that you will rarely encounter.