Created a 'DB optimization' topic, with cross-refs to relevant sections.

FAQ: Databases and models

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

.. _faq-see-raw-sql-queries:

How can I see the raw SQL queries Django is running?

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

    * **Other:**

      :ref:`Supported databases <ref-databases>` |

      :ref:`Legacy databases <howto-legacy-databases>` |

      :ref:`Providing initial data <howto-initial-data>`

      :ref:`Providing initial data <howto-initial-data>` |

      :ref:`Optimize database access <topics-db-optimization>`

The template layer

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

      iterating over a ``QuerySet`` will take advantage of your database to

      load data and instantiate objects only as you need them.

    * **bool().** Testing a ``QuerySet`` in a boolean context, such as using

      ``bool()``, ``or``, ``and`` or an ``if`` statement, will cause the query

      to be executed. If there is at least one result, the ``QuerySet`` is

      ``True``, otherwise ``False``. For example::

          if Entry.objects.filter(headline="Test"):

             print "There is at least one Entry with the headline Test"

      Note: *Don't* use this if all you want to do is determine if at least one

      result exists, and don't need the actual objects. It's more efficient to

      use ``exists()`` (see below).

.. _pickling QuerySets:

Pickling QuerySets

ordering was undefined prior to calling ``reverse()``, and will remain

undefined afterward).

.. _querysets-distinct:

.. _queryset-distinct:

``distinct()``

~~~~~~~~~~~~~~

    ``values()`` call.

.. _queryset-values:

``values(*fields)``

~~~~~~~~~~~~~~~~~~~

Both the ``depth`` argument and the ability to specify field names in the call

to ``select_related()`` are new in Django version 1.0.

.. _extra:

.. _queryset-extra:

``extra(select=None, where=None, params=None, tables=None, order_by=None, select_params=None)``

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you pass ``in_bulk()`` an empty list, you'll get an empty dictionary.

.. _queryset-iterator:

``iterator()``

~~~~~~~~~~~~~~

Evaluates the ``QuerySet`` (by performing the query) and returns an

`iterator`_ over the results. A ``QuerySet`` typically reads all of

its results and instantiates all of the corresponding objects the

first time you access it; ``iterator()`` will instead read results and

instantiate objects in discrete chunks, yielding them one at a

time. For a ``QuerySet`` which returns a large number of objects, this

often results in better performance and a significant reduction in

memory use.

`iterator`_ over the results. A ``QuerySet`` typically caches its

results internally so that repeated evaluations do not result in

additional queries; ``iterator()`` will instead read results directly,

without doing any caching at the ``QuerySet`` level. For a

``QuerySet`` which returns a large number of objects, this often

results in better performance and a significant reduction in memory

Note that using ``iterator()`` on a ``QuerySet`` which has already

been evaluated will force it to evaluate again, repeating the query.

query.

This behavior is the same as that noted in the queryset documentation for

:ref:`distinct() <querysets-distinct>` and the general rule is the same:

:ref:`distinct() <queryset-distinct>` and the general rule is the same:

normally you won't want extra columns playing a part in the result, so clear

out the ordering, or at least make sure it's restricted only to those fields

you also select in a ``values()`` call.

   sql

   transactions

   multi-db

   optimization