A hash index contains hashed values of the indexed field:

> db.books.createIndex( { name: "hashed" } )

This will create a hashed index on the name of each book of our books collection.

A hashed index is ideal for equality matches but cannot work with range queries. If we want to perform range queries on fields, we can create a regular index (or a compound index containing the field) and also a hash index for equality matches. Hashed indexes are used internally by MongoDB for hashed-based sharding as we will discuss in Chapter 11, Sharding. Hashed indexes truncate floating point fields to integers. Floating points should be avoided for hashed fields wherever possible.