10.1 Query Processing and Query Optimization
In Chapter 5, we saw that some SQL queries can be expressed in different ways. For example, in a SELECT statement involving two or more tables, we can sometimes choose between a join and a subquery. Similarly, in Chapter 4, we considered that different sequences of relational algebra commands that produce equivalent results. We noted in passing certain formulations of these queries are more efficient than others. For example, it is more efficient to perform a natural join instead of a Cartesian product followed by a selection. In this chapter, we examine relative efficiency and discuss some techniques for improving the efficiency of queries.
When a database management system (DBMS) receives a high-level ...
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