Understanding additional b-tree features

In PostgreSQL, indexing is a large field and covers many aspects of database work. As I have outlined in this book already, indexing is the key to performance. There is no good performance without proper indexing. Therefore, it is worth inspecting these indexing- related features in more detail.

Combined indexes

In my job, as a professional PostgreSQL support vendor, I am often asked about the difference between combined and individual indexes. In this section, I will try to shed some light on this question.

The general rule is this if a single index can answer your question, it is usually the best choice. However, you cannot index all possible combinations of fields people are filtering on. What you can do is use the properties of combined indexes to achieve as much gain as possible.

Let's suppose we have a table containing three columns: postal_code, last_name, and first_name. A telephone book would make use of a combined index like this. You will see...