============================
Database access optimization
============================

Django's database layer provides various ways to help developers get the most
out of their databases. This document gathers together links to the relevant
documentation, and adds various tips, organized under a number of headings that
outline the steps to take when attempting to optimize your database usage.

Profile first
=============

As general programming practice, this goes without saying. Find out :ref:`what
queries you are doing and what they are costing you <faq-see-raw-sql-queries>`.
Use :meth:`.QuerySet.explain` to understand how specific ``QuerySet``\s are
executed by your database. You may also want to use an external project like
:pypi:`django-debug-toolbar`, or a tool that monitors your database directly.

Remember that you may be optimizing for speed or memory or both, depending on
your requirements. Sometimes optimizing for one will be detrimental to the
other, but sometimes they will help each other. Also, work that is done by the
database process might not have the same cost (to you) as the same amount of
work done in your Python process. It is up to you to decide what your
priorities are, where the balance must lie, and profile all of these as
required since this will depend on your application and server.

With everything that follows, remember to profile after every change to ensure
that the change is a benefit, and a big enough benefit given the decrease in
readability of your code. **All** of the suggestions below come with the caveat
that in your circumstances the general principle might not apply, or might even
be reversed.

Use standard DB optimization techniques
=======================================

...including:

* Indexes_. This is a number one priority, *after* you have determined from
  profiling what indexes should be added. Use
  :attr:`Meta.indexes <django.db.models.Options.indexes>` or
  :attr:`Field.db_index <django.db.models.Field.db_index>` to add these from
  Django. Consider adding indexes to fields that you frequently query using
  :meth:`~django.db.models.query.QuerySet.filter`,
  :meth:`~django.db.models.query.QuerySet.exclude`,
  :meth:`~django.db.models.query.QuerySet.order_by`, etc. as indexes may help
  to speed up lookups. Note that determining the best indexes is a complex
  database-dependent topic that will depend on your particular application.
  The overhead of maintaining an index may outweigh any gains in query speed.

.. _Indexes: https://en.wikipedia.org/wiki/Database_index

* Appropriate use of field types.

We will assume you have done the things listed above. The rest of this document
focuses on how to use Django in such a way that you are not doing unnecessary
work. This document also does not address other optimization techniques that
apply to all expensive operations, such as :doc:`general purpose caching
</topics/cache>`.

Understand ``QuerySet``\s
=========================

Understanding :doc:`QuerySets </ref/models/querysets>` is vital to getting good
performance with simple code. In particular:

Understand ``QuerySet`` evaluation
----------------------------------

To avoid performance problems, it is important to understand:

* that :ref:`QuerySets are lazy <querysets-are-lazy>`.

* when :ref:`they are evaluated <when-querysets-are-evaluated>`.

* how :ref:`the data is held in memory <caching-and-querysets>`.

Understand cached attributes
----------------------------

As well as caching of the whole ``QuerySet``, there is caching of the result of
attributes on ORM objects. In general, attributes that are not callable will be
cached. For example, assuming the :ref:`example blog models
<queryset-model-example>`:

.. code-block:: pycon

  >>> entry = Entry.objects.get(id=1)
  >>> entry.blog  # Blog object is retrieved at this point
  >>> entry.blog  # cached version, no DB access

But in general, callable attributes cause DB lookups every time:

.. code-block:: pycon

  >>> entry = Entry.objects.get(id=1)
  >>> entry.authors.all()  # query performed
  >>> entry.authors.all()  # query performed again

Be careful when reading template code - the template system does not allow use
of parentheses, but will call callables automatically, hiding the above
distinction.

Be careful with your own custom properties - it is up to you to implement
caching when required, for example using the
:class:`~django.utils.functional.cached_property` decorator.

Use the ``with`` template tag
-----------------------------

To make use of the caching behavior of ``QuerySet``, you may need to use the
:ttag:`with` template tag.

Use ``iterator()``
------------------

When you have a lot of objects, the caching behavior of the ``QuerySet`` can
cause a large amount of memory to be used. In this case,
:meth:`~django.db.models.query.QuerySet.iterator` may help.

Use ``explain()``
-----------------

:meth:`.QuerySet.explain` gives you detailed information about how the database
executes a query, including indexes and joins that are used. These details may
help you find queries that could be rewritten more efficiently, or identify
indexes that could be added to improve performance.

Do database work in the database rather than in Python
======================================================

For instance:

* At the most basic level, use :ref:`filter and exclude <queryset-api>` to do
  filtering in the database.

* Use :ref:`f-expressions` to filter based on other fields within the same
  model.

* Use :doc:`annotate to do aggregation in the database
  </topics/db/aggregation>`.

If these aren't enough to generate the SQL you need:

Use ``RawSQL``
--------------

A less portable but more powerful method is the
:class:`~django.db.models.expressions.RawSQL` expression, which allows some SQL
to be explicitly added to the query. If that still isn't powerful enough:

Use raw SQL
-----------

Write your own :doc:`custom SQL to retrieve data or populate models
</topics/db/sql>`. Use ``django.db.connection.queries`` to find out what Django
is writing for you and start from there.

Retrieve individual objects using a unique, indexed column
==========================================================

There are two reasons to use a column with
:attr:`~django.db.models.Field.unique` or
:attr:`~django.db.models.Field.db_index` when using
:meth:`~django.db.models.query.QuerySet.get` to retrieve individual objects.
First, the query will be quicker because of the underlying database index.
Also, the query could run much slower if multiple objects match the lookup;
having a unique constraint on the column guarantees this will never happen.

So using the :ref:`example blog models <queryset-model-example>`:

.. code-block:: pycon

  >>> entry = Entry.objects.get(id=10)

will be quicker than:

.. code-block:: pycon

  >>> entry = Entry.objects.get(headline="News Item Title")

because ``id`` is indexed by the database and is guaranteed to be unique.

Doing the following is potentially quite slow:

.. code-block:: pycon

  >>> entry = Entry.objects.get(headline__startswith="News")

First of all, ``headline`` is not indexed, which will make the underlying
database fetch slower.

Second, the lookup may return multiple results. Even though Django applies a
limit of 21 objects when using :meth:`~django.db.models.query.QuerySet.get`,
the database may still need to scan everything to evaluate a non-unique
condition. Narrowing your query up-front is more efficient.

Retrieve related objects efficiently
====================================

Generally, accessing the database multiple times to retrieve different parts
of a single "set" of data is less efficient than retrieving it all in one
query. This is particularly important if you have a query that is executed in a
loop, and could therefore end up doing many database queries, when only one
is needed. Below are some techniques to combine queries for efficiency.

Use the ``FETCH_PEERS`` fetch mode
----------------------------------

Use the :attr:`~django.db.models.FETCH_PEERS` fetch mode to make on-demand
field access more efficient with bulk-fetching. Enable it for all usage of
your models :ref:`with a custom manager <fetch-modes-custom-manager>`.

Using this fetch mode is easier than declaring fields to fetch with
:meth:`~django.db.models.query.QuerySet.select_related` or
:meth:`~django.db.models.query.QuerySet.prefetch_related`, especially when it's
hard to predict which fields will be accessed.

Use ``QuerySet.select_related()`` and ``prefetch_related()``
------------------------------------------------------------

When the :attr:`~django.db.models.FETCH_PEERS` fetch mode is not appropriate or
efficient enough, use :meth:`~django.db.models.query.QuerySet.select_related`
and :meth:`~django.db.models.query.QuerySet.prefetch_related`. Understand their
documentation thoroughly and apply them where needed.

It may be useful to apply these methods in :doc:`managers and default managers
</topics/db/managers>`. Be aware when your manager is and is not used; for
example, related object access :ref:`uses the base manager
<managers-for-related-objects>` rather than the default manager.

Use ``prefetch_related_objects()``
----------------------------------

Where :attr:`~django.db.models.query.QuerySet.prefetch_related` would be useful
after the queryset has been evaluated, use
:func:`~django.db.models.prefetch_related_objects` to execute an extra
prefetch.

Don't retrieve things you don't need
====================================

Use ``QuerySet.values()`` and ``values_list()``
-----------------------------------------------

When you only want a ``dict`` or ``list`` of values, and don't need ORM model
objects, make appropriate usage of
:meth:`~django.db.models.query.QuerySet.values`.
These can be useful for replacing model objects in template code - as long as
the dicts you supply have the same attributes as those used in the template,
you are fine.

Use ``QuerySet.defer()`` and ``only()``
---------------------------------------

Use :meth:`~django.db.models.query.QuerySet.defer` and
:meth:`~django.db.models.query.QuerySet.only` if there are database columns
you know that you won't need (or won't need in most cases) to avoid loading
them. Note that if you *do* use them, the ORM will have to go and get them in
a separate query, making this a pessimization if you use it inappropriately.

Don't be too aggressive in deferring fields without profiling as the database
has to read most of the non-text, non-``VARCHAR`` data from the disk for a
single row in the results, even if it ends up only using a few columns. The
``defer()`` and ``only()`` methods are most useful when you can avoid loading a
lot of text data or for fields that might take a lot of processing to convert
back to Python. As always, profile first, then optimize.

Use ``QuerySet.contains(obj)``
------------------------------

...if you only want to find out if ``obj`` is in the queryset, rather than
``if obj in queryset``.

Use ``QuerySet.count()``
------------------------

...if you only want the count, rather than doing ``len(queryset)``.

Use ``QuerySet.exists()``
-------------------------

...if you only want to find out if at least one result exists, rather than ``if
queryset``.

But:

.. _overuse_of_count_and_exists:

Don't overuse ``contains()``, ``count()``, and ``exists()``
-----------------------------------------------------------

If you are going to need other data from the QuerySet, evaluate it immediately.

For example, assuming a ``Group`` model that has a many-to-many relation to
``User``, the following code is optimal::

    members = group.members.all()

    if display_group_members:
        if members:
            if current_user in members:
                print("You and", len(members) - 1, "other users are members of this group.")
            else:
                print("There are", len(members), "members in this group.")

            for member in members:
                print(member.username)
        else:
            print("There are no members in this group.")

It is optimal because:

#. Since QuerySets are lazy, this does no database queries if
   ``display_group_members`` is ``False``.

#. Storing ``group.members.all()`` in the ``members`` variable allows its
   result cache to be reused.

#. The line ``if members:`` causes ``QuerySet.__bool__()`` to be called, which
   causes the ``group.members.all()`` query to be run on the database. If there
   aren't any results, it will return ``False``, otherwise ``True``.

#. The line ``if current_user in members:`` checks if the user is in the result
   cache, so no additional database queries are issued.

#. The use of ``len(members)`` calls ``QuerySet.__len__()``, reusing the result
   cache, so again, no database queries are issued.

#. The ``for member`` loop iterates over the result cache.

In total, this code does either one or zero database queries. The only
deliberate optimization performed is using the ``members`` variable. Using
``QuerySet.exists()`` for the ``if``, ``QuerySet.contains()`` for the ``in``,
or ``QuerySet.count()`` for the count would each cause additional queries.

Use ``QuerySet.update()`` and ``delete()``
------------------------------------------

Rather than retrieve a load of objects, set some values, and save them
individual, use a bulk SQL UPDATE statement, via :ref:`QuerySet.update()
<topics-db-queries-update>`. Similarly, do :ref:`bulk deletes
<topics-db-queries-delete>` where possible.

Note, however, that these bulk update methods cannot call the ``save()`` or
``delete()`` methods of individual instances, which means that any custom
behavior you have added for these methods will not be executed, including
anything driven from the normal database object :doc:`signals </ref/signals>`.

Use foreign key values directly
-------------------------------

If you only need a foreign key value, use the foreign key value that is already
on the object you've got, rather than getting the whole related object and
taking its primary key. i.e. do::

   entry.blog_id

instead of::

   entry.blog.id

Don't order results if you don't care
-------------------------------------

Ordering is not free; each field to order by is an operation the database must
perform. If a model has a default ordering (:attr:`Meta.ordering
<django.db.models.Options.ordering>`) and you don't need it, remove
it on a ``QuerySet`` by calling
:meth:`~django.db.models.query.QuerySet.order_by` with no parameters.

Adding an index to your database may help to improve ordering performance.

Use bulk methods
================

Use bulk methods to reduce the number of SQL statements.

Create in bulk
--------------

When creating objects, where possible, use the
:meth:`~django.db.models.query.QuerySet.bulk_create` method to reduce the
number of SQL queries. For example::

    Entry.objects.bulk_create(
        [
            Entry(headline="This is a test"),
            Entry(headline="This is only a test"),
        ]
    )

...is preferable to::

    Entry.objects.create(headline="This is a test")
    Entry.objects.create(headline="This is only a test")

Note that :meth:`~django.db.models.query.QuerySet.bulk_create` has several
caveats, so ensure it's appropriate for your use case.

Update in bulk
--------------

When updating objects, where possible, use the
:meth:`~django.db.models.query.QuerySet.bulk_update` method to reduce the
number of SQL queries. Given a list or queryset of objects::

    entries = Entry.objects.bulk_create(
        [
            Entry(headline="This is a test"),
            Entry(headline="This is only a test"),
        ]
    )

The following example::

    entries[0].headline = "This is not a test"
    entries[1].headline = "This is no longer a test"
    Entry.objects.bulk_update(entries, ["headline"])

...is preferable to::

    entries[0].headline = "This is not a test"
    entries[0].save()
    entries[1].headline = "This is no longer a test"
    entries[1].save()

Note that :meth:`~django.db.models.query.QuerySet.bulk_update` has several
caveats, so ensure it's appropriate for your use case.

Insert in bulk
--------------

When inserting objects into :class:`ManyToManyFields
<django.db.models.ManyToManyField>`, use
:meth:`~django.db.models.fields.related.RelatedManager.add` with multiple
objects to reduce the number of SQL queries. For example::

    my_band.members.add(me, my_friend)

...is preferable to::

    my_band.members.add(me)
    my_band.members.add(my_friend)

...where ``Band`` and ``Artist`` are models with a many-to-many relationship.

When inserting different pairs of objects into
:class:`~django.db.models.ManyToManyField` or when the custom
:attr:`~django.db.models.ManyToManyField.through` table is defined, use
:meth:`~django.db.models.query.QuerySet.bulk_create` method to reduce the
number of SQL queries. For example::

    PizzaToppingRelationship = Pizza.toppings.through
    PizzaToppingRelationship.objects.bulk_create(
        [
            PizzaToppingRelationship(pizza=my_pizza, topping=pepperoni),
            PizzaToppingRelationship(pizza=your_pizza, topping=pepperoni),
            PizzaToppingRelationship(pizza=your_pizza, topping=mushroom),
        ],
        ignore_conflicts=True,
    )

...is preferable to::

    my_pizza.toppings.add(pepperoni)
    your_pizza.toppings.add(pepperoni, mushroom)

...where ``Pizza`` and ``Topping`` have a many-to-many relationship. Note that
there are a number of :meth:`caveats to this method
<django.db.models.query.QuerySet.bulk_create>`, so make sure it's appropriate
for your use case.

Remove in bulk
--------------

When removing objects from :class:`ManyToManyFields
<django.db.models.ManyToManyField>`, use
:meth:`~django.db.models.fields.related.RelatedManager.remove` with multiple
objects to reduce the number of SQL queries. For example::

    my_band.members.remove(me, my_friend)

...is preferable to::

    my_band.members.remove(me)
    my_band.members.remove(my_friend)

...where ``Band`` and ``Artist`` are models with a many-to-many relationship.

When removing multiple many-to-many relationships involving several instances
of the related models, use the :meth:`~django.db.models.query.QuerySet.delete`
method on a filtered queryset of the field's
:attr:`~django.db.models.ManyToManyField.through` model. By combining multiple
conditions with :ref:`q-objects`, you can delete several relationships in a
single query. For example::

    from django.db.models import Q

    PizzaToppingRelationship = Pizza.toppings.through
    PizzaToppingRelationship.objects.filter(
        Q(pizza=my_pizza, topping=pepperoni)
        | Q(pizza=your_pizza, topping=pepperoni)
        | Q(pizza=your_pizza, topping=mushroom)
    ).delete()

...is preferable to::

    my_pizza.toppings.remove(pepperoni)
    your_pizza.toppings.remove(pepperoni, mushroom)

...where ``Pizza`` and ``Topping`` have a many-to-many relationship.
