Fixed #20877 -- added a performance optimization guide

* :doc:`"Local flavor" <topics/localflavor>`

* :doc:`Time zones </topics/i18n/timezones>`

Performance and optimization

============================

There are a variety of techniques and tools that can help get your code running

more efficiently - faster, and using fewer system resources.

* :doc:`Performance and optimization overview <topics/performance>`

Python compatibility

====================

    For cases like this, use the ``django.utils.functional.allow_lazy()``

    decorator. It modifies the function so that *if* it's called with a lazy

    translation as the first argument, the function evaluation is delayed until it

    needs to be converted to a string.

    translation as one of its arguments, the function evaluation is delayed

    until it needs to be converted to a string.

    For example::

.. _`Cache-Control spec`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9

Other optimizations

===================

Django comes with a few other pieces of middleware that can help optimize your

site's performance:

* ``django.middleware.http.ConditionalGetMiddleware`` adds support for

  modern browsers to conditionally GET responses based on the ``ETag``

  and ``Last-Modified`` headers.

* :class:`django.middleware.gzip.GZipMiddleware` compresses responses for all

  modern browsers, saving bandwidth and transfer time. Be warned, however,

  that compression techniques like ``GZipMiddleware`` are subject to attacks.

  See the warning in :class:`~django.middleware.gzip.GZipMiddleware` for

  details.

Order of MIDDLEWARE_CLASSES

===========================

distinction.

Be careful with your own custom properties - it is up to you to implement

caching.

caching when required, for example using the

:class:`~django.utils.functional.cached_property` decorator.

Use the ``with`` template tag

-----------------------------

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

  filtering in the database.

* Use :class:`F expressions <django.db.models.F>` to do filtering

  against other fields within the same model.

* Use :class:`F expressions <django.db.models.F>` to filter

  based on other fields within the same model.

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

* Use :doc:`annotate to do aggregation in the database

  </topics/db/aggregation>`.

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

But:

.. _overuse_of_count_and_exists:

Don't overuse ``count()`` and ``exists()``

------------------------------------------

   pagination

   python3

   security

   performance

   serialization

   settings

   signals