django/docs/ref/contrib/postgres/lookups.txt

===========================
PostgreSQL specific lookups
===========================

Trigram similarity
==================

.. fieldlookup:: trigram_similar

The ``trigram_similar`` lookup allows you to perform trigram lookups,
measuring the number of trigrams (three consecutive characters) shared, using a
dedicated PostgreSQL extension. A trigram lookup is given an expression and
returns results that have a similarity measurement greater than the current
similarity threshold.

To use it, add ``'django.contrib.postgres'`` in your :setting:`INSTALLED_APPS`
and activate the `pg_trgm extension
<https://www.postgresql.org/docs/current/pgtrgm.html>`_ on PostgreSQL. You can
install the extension using the
:class:`~django.contrib.postgres.operations.TrigramExtension` migration
operation.

The ``trigram_similar`` lookup can be used on
:class:`~django.db.models.CharField` and :class:`~django.db.models.TextField`::

    >>> City.objects.filter(name__trigram_similar="Middlesborough")
    ['<City: Middlesbrough>']

``Unaccent``
============

.. fieldlookup:: unaccent

The ``unaccent`` lookup allows you to perform accent-insensitive lookups using
a dedicated PostgreSQL extension.

This lookup is implemented using :class:`~django.db.models.Transform`, so it
can be chained with other lookup functions. To use it, you need to add
``'django.contrib.postgres'`` in your :setting:`INSTALLED_APPS` and activate
the `unaccent extension on PostgreSQL`_. The
:class:`~django.contrib.postgres.operations.UnaccentExtension` migration
operation is available if you want to perform this activation using migrations).

.. _unaccent extension on PostgreSQL: https://www.postgresql.org/docs/current/unaccent.html

The ``unaccent`` lookup can be used on
:class:`~django.db.models.CharField` and :class:`~django.db.models.TextField`::

    >>> City.objects.filter(name__unaccent="México")
    ['<City: Mexico>']

    >>> User.objects.filter(first_name__unaccent__startswith="Jerem")
    ['<User: Jeremy>', '<User: Jérémy>', '<User: Jérémie>', '<User: Jeremie>']

.. warning::

    ``unaccent`` lookups should perform fine in most use cases. However, queries
    using this filter will generally perform full table scans, which can be slow
    on large tables. In those cases, using dedicated full text indexing tools
    might be appropriate.