A hashed 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 it cannot work with range queries. If we want to perform a range of queries on fields, we can create a regular index (or a compound index containing the field), and also a hashed index for equality matches. Hashed indexes are used internally by MongoDB for hash-based sharding, as we will discuss in Chapter 13, Sharding. Hashed indexes truncate floating point fields to integers. Floating points should be avoided for hashed fields wherever possible.