Chapter 8. Indexing and query optimization

Indexes are enormously important. With the right indexes in place, MongoDB can use its hardware efficiently and serve your application’s queries quickly. But the wrong indexes produce the opposite result: slow queries, slow writes, and poorly utilized hardware. It stands to reason that anyone wanting to use MongoDB effectively must understand indexing.

But for many developers, indexes are a topic shrouded in mystery. This need not be the case. Once you’ve finished this chapter, you should have a good mental model for thinking clearly about indexes. To introduce the concepts of indexing, we’ll begin with a modest thought experiment. We’ll then explore some core indexing concepts and provide an overview of the B-tree data structure underlying MongoDB indexes.

Then it’s on to indexing in practice. We’ll discuss unique, sparse, and multikey indexes, and provide a number of pointers on index administration. Next, we’ll delve into query optimization, describing how to use explain() and work harmoniously with the query optimizer.

8.1. Indexing theory

8.2. Indexing in practice

8.3. Query optimization

8.4. Summary