MongoDB: The Definitive Guide, 2nd Edition

Chapter 5. Indexing

This chapter introduces MongoDB’s indexing, which allows you to optimize your queries and is even required for certain types of queries:

• What indexing is and why you’d want to use it

• How to choose which fields to index

• How to enforce and evaluate index usage

• Administrative details on creating and removing indexes

Choosing the right indexes for your collections is critical to performance.

Introduction to Indexing

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 that points to the content, which allows it to query orders of magnitude faster.

A query that does not use an index is called a table scan (a term inherited from relational databases), 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 start at page 1 and read through the whole thing. In general, you want to avoid making the server do table scans because it is very slow for large collections.

For example, let’s 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.insert(
...         {
...              "i" : i, 
...              "username" : "user"+i,
...              "age" : Math.floor(Math.random()*120), 
...              "created" : new Date()
...         }
...     );
... }

If we do a query on this collection, we can use the explain() function ...