django1/docs/ref/models/indexes.txt

=====================
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, opclasses=(), condition=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.

``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 (_).

.. admonition:: Partial indexes in abstract base classes

    You must always specify a unique name for an index. As such, you
    cannot normally specify a partial index on an abstract base class, since
    the :attr:`Meta.indexes <django.db.models.Options.indexes>` option is
    inherited by subclasses, with exactly the same values for the attributes
    (including ``name``) each time. To work around name collisions, part of the
    name may contain ``'%(app_label)s'`` and ``'%(class)s'``, which are
    replaced, respectively, by the lowercased app label and class name of the
    concrete model. For example ``Index(fields=['title'],
    name='%(app_label)s_%(class)s_title_index')``.

``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`.

``opclasses``
-------------

.. attribute:: Index.opclasses

The names of the `PostgreSQL operator classes
<https://www.postgresql.org/docs/current/indexes-opclass.html>`_ to use for
this index. If you require a custom operator class, you must provide one for
each field in the index.

For example, ``GinIndex(name='json_index', fields=['jsonfield'],
opclasses=['jsonb_path_ops'])`` creates a gin index on ``jsonfield`` using
``jsonb_path_ops``.

``opclasses`` are ignored for databases besides PostgreSQL.

:attr:`Index.name` is required when using ``opclasses``.

``condition``
-------------

.. attribute:: Index.condition

If the table is very large and your queries mostly target a subset of rows,
it may be useful to restrict an index to that subset. Specify a condition as a
:class:`~django.db.models.Q`. For example, ``condition=Q(pages__gt=400)``
indexes records with more than 400 pages.

:attr:`Index.name` is required when using ``condition``.

.. admonition:: Restrictions on PostgreSQL

    PostgreSQL requires functions referenced in the condition to be marked as
    IMMUTABLE. Django doesn't validate this but PostgreSQL will error. This
    means that functions such as :ref:`date-functions` and
    :class:`~django.db.models.functions.Concat` aren't accepted. If you store
    dates in :class:`~django.db.models.DateTimeField`, comparison to
    :class:`~datetime.datetime` objects may require the ``tzinfo`` argument
    to be provided because otherwise the comparison could result in a mutable
    function due to the casting Django does for :ref:`lookups <field-lookups>`.

.. admonition:: Restrictions on SQLite

    SQLite `imposes restrictions <https://www.sqlite.org/partialindex.html>`_
    on how a partial index can be constructed.

.. admonition:: Oracle

    Oracle does not support partial indexes. Instead, partial indexes can be
    emulated using functional indexes. Use a :doc:`migration
    </topics/migrations>` to add the index using :class:`.RunSQL`.

.. admonition:: MySQL and MariaDB

    The ``condition`` argument is ignored with MySQL and MariaDB as neither
    supports conditional indexes.