Hashed Index¶

New in version 2.4.

Hashed indexes maintain entries with hashes of the values of the indexed field. The hashing function collapses embedded documents and computes the hash for the entire value but does not support multi-key (i.e. arrays) indexes.

Hashed indexes support sharding a collection using a hashed shard key. Using a hashed shard key to shard a collection ensures a more even distribution of data. See Shard a Collection Using a Hashed Shard Key for more details.

MongoDB can use the hashed index to support equality queries, but hashed indexes do not support range queries.

You may not create compound indexes that have hashed index fields or specify a unique constraint on a hashed index; however, you can create both a hashed index and an ascending/descending (i.e. non-hashed) index on the same field: MongoDB will use the scalar index for range queries.

Warning

MongoDB hashed indexes truncate floating point numbers to 64-bit integers before hashing. For example, a hashed index would store the same value for a field that held a value of 2.3, 2.2, and 2.9. To prevent collisions, do not use a hashed index for floating point numbers that cannot be reliably converted to 64-bit integers (and then back to floating point). MongoDB hashed indexes do not support floating point values larger than 2⁵³.

Create a hashed index using an operation that resembles the following:

db.active.createIndex( { a: "hashed" } )

This operation creates a hashed index for the active collection on the a field.

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