MongoDB: The Definitive Guide, 3rd Edition

Chapter 5. Indexes

This chapter introduces MongoDB indexes. Indexes enable you to perform queries efficiently. They’re an important part of application development and are even required for certain types of queries. In this chapter we will cover:

• What indexes are and why you’d want to use them

• How to choose which fields to index

• How to enforce and evaluate index usage

• Administrative details on creating and removing indexes

As you’ll see, choosing the right indexes for your collections is critical to performance.

Introduction to Indexes

A database index is similar to a book’s index. Instead of looking through the whole book, the database takes a shortcut and just looks at an ordered list with references to the content. This allows MongoDB to query orders of magnitude faster.

A query that does not use an index is called a collection scan, which means that the server has to “look through the whole book” to find a query’s results. This process is basically what you’d do if you were looking for information in a book without an index: you’d start at page 1 and read through the whole thing. In general, you want to avoid making the server do collection scans because the process is very slow for large collections.

Let’s look at an example. To get started, we’ll create a collection with 1 million documents in it (or 10 million or 100 million, if you have the patience):

> for (i=0; i<1000000; i++) {
...     db.users.insertOne(
...         {
...              "i" : i, 
...              "username" : "user"+i,
...              "age" : Math.floor(Math.random ...