High Performance MySQL, 3rd Edition

Chapter 6. Query Performance Optimization

In the previous chapters we explained schema optimization and indexing, which are necessary for high performance. But they aren’t enough—you also need to design your queries well. If your queries are bad, even the best-designed schema and indexes will not perform well.

Query optimization, index optimization, and schema optimization go hand in hand. As you gain experience writing queries in MySQL, you will learn how to design tables and indexes to support efficient queries. Similarly, what you learn about optimal schema design will influence the kinds of queries you write. This process takes time, so we encourage you to refer back to these three chapters as you learn more.

This chapter begins with general query design considerations—the things you should consider first when a query isn’t performing well. We then dig much deeper into query optimization and server internals. We show you how to find out how MySQL executes a particular query, and you’ll learn how to change the query execution plan. Finally, we’ll look at some places MySQL doesn’t optimize queries well and explore query optimization patterns that help MySQL execute queries more efficiently.

Our goal is to help you understand deeply how MySQL really executes queries, so you can reason about what is efficient or inefficient, exploit MySQL’s strengths, and avoid its weaknesses.

Why Are Queries Slow?

Before trying to write fast queries, remember that it’s all about response time. Queries ...