create-a-hashed-index.txt

.. index:: index; hashed
.. _index-hashed-index:

=====================
Create a Hashed Index
=====================

.. default-domain:: mongodb

.. contents:: On this page
   :local:
   :backlinks: none
   :depth: 1
   :class: singlecol

.. versionadded:: 2.4

:ref:`Hashed indexes <index-type-hashed>` compute a hash of the value
of a field in a collection and index the hashed value. These indexes
permit equality queries and may be suitable shard keys for some
collections.


.. include:: /includes/tip-applications-do-not-need-to-compute-hashes.rst

.. see:: :ref:`sharding-hashed-sharding` for more information about hashed
   indexes in sharded clusters, as well as :doc:`/core/indexes` and
   :doc:`/administration/indexes` for more information about indexes.

Procedure
---------

To create a :ref:`hashed index <index-type-hashed>`, specify
``hashed`` as the value of the index key, as in the following
example:

.. example:: Specify a hashed index on ``_id``

   .. code-block:: javascript

      db.collection.createIndex( { _id: "hashed" } )

Considerations
--------------

MongoDB supports ``hashed`` indexes of any single 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.

You may not create compound indexes that have ``hashed`` index fields.