In Chapter 5, Log Files and System Statistics, you learned how to read system statistics and how to make use of what PostgreSQL provides. Armed with this knowledge, this chapter is all about good query performance. 

In this chapter, you will learn about the following topics:

• Learning what the optimizer does
• Understanding execution plans
• Understanding and fixing joins
• Enabling and disabling optimizer settings
• Partitioning data
• Adjusting parameters for good query performance
• Making use of parallel queries
• Introducing JIT compilation

By the end of this chapter, we will be able to write better and faster queries. If the queries still aren't very good, we should be able to understand why this is the case. We will also be able to use the new techniques we will have learned about to partition data.