Physical Database Design

32 CHAPTER 3 Query Optimization and Plan Selection

execution plan would be best. We focus here on the practical tradeoff analysis needed to

find the best query execution plan to illustrate the process.

3.1 Query Processing and Optimization

The basic steps of query processing are:

1. Scanning, parsing, and decomposition of an SQL query. This step checks for correct

SQL query syntax and generates appropriate error messages when necessary. The

output of this step is an intermediate form of the query known as a query tree or

query execution plan.

2. Query optimization. This step includes both local and global optimization. Glo-

bal optimization determines the order of joins and the order of selections and

projections relative to the joins. It also involves ...