=====================
Model index reference
=====================

.. module:: django.db.models.indexes

.. currentmodule:: django.db.models

Index classes ease creating database indexes. They can be added using the
:attr:`Meta.indexes <django.db.models.Options.indexes>` option. This document
explains the API references of :class:`Index` which includes the `index
options`_.

.. admonition:: Referencing built-in indexes

    Indexes are defined in ``django.db.models.indexes``, but for convenience
    they're imported into :mod:`django.db.models`. The standard convention is
    to use ``from django.db import models`` and refer to the indexes as
    ``models.<IndexClass>``.

``Index`` options
=================

.. class:: Index(fields=(), name=None, db_tablespace=None)

    Creates an index (B-Tree) in the database.

``fields``
----------

.. attribute:: Index.fields

A list or tuple of the name of the fields on which the index is desired.

By default, indexes are created with an ascending order for each column. To
define an index with a descending order for a column, add a hyphen before the
field's name.

For example ``Index(fields=['headline', '-pub_date'])`` would create SQL with
``(headline, pub_date DESC)``. Index ordering isn't supported on MySQL. In that
case, a descending index is created as a normal index.

.. versionchanged:: 2.1

    Older versions don't accept a tuple.

``name``
--------

.. attribute:: Index.name

The name of the index. If ``name`` isn't provided Django will auto-generate a
name. For compatibility with different databases, index names cannot be longer
than 30 characters and shouldn't start with a number (0-9) or underscore (_).

``db_tablespace``
-----------------

.. attribute:: Index.db_tablespace

The name of the :doc:`database tablespace </topics/db/tablespaces>` to use for
this index. For single field indexes, if ``db_tablespace`` isn't provided, the
index is created in the ``db_tablespace`` of the field.

If :attr:`.Field.db_tablespace` isn't specified (or if the index uses multiple
fields), the index is created in tablespace specified in the
:attr:`~django.db.models.Options.db_tablespace` option inside the model's
``class Meta``. If neither of those tablespaces are set, the index is created
in the same tablespace as the table.

.. seealso::

    For a list of PostgreSQL-specific indexes, see
    :mod:`django.contrib.postgres.indexes`.