Fixed #24190 -- Clarified len(queryset)

* **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it.

  This, as you might expect, returns the length of the result list.

  Note: *Don't* use ``len()`` on ``QuerySet``\s if all you want to do is

  determine the number of records in the set. It's much more efficient to

  handle a count at the database level, using SQL's ``SELECT COUNT(*)``,

  and Django provides a ``count()`` method for precisely this reason. See

  ``count()`` below.

  Note: If you only need to determine the number of records in the set (and

  don't need the actual objects), it's much more efficient to handle a count

  at the database level using SQL's ``SELECT COUNT(*)``. Django provides a

  :meth:`~QuerySet.count` method for precisely this reason.

* **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on

  it. 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 :meth:`~QuerySet.exists` (see below).

  Note: If you only want to determine if at least one result exists (and don't

  need the actual objects), it's more efficient to use :meth:`~QuerySet.exists`.

.. _pickling QuerySets:

is an underlying implementation quirk that shouldn't pose any real-world

problems.

Note that if you want the number of items in a ``QuerySet`` and are also

retrieving model instances from it (for example, by iterating over it), it's

probably more efficient to use ``len(queryset)`` which won't cause an extra

database query like ``count()`` would.

in_bulk

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