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Fixed #20513 - Expanded docs on QuerySet caching.

Thanks seddonym.
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timgraham committed May 28, 2013
1 parent 33dd8f5 commit 8365d76da01af1d4391cba32d62178791d074b06
Showing with 40 additions and 3 deletions.
  1. +40 −3 docs/topics/db/queries.txt
@@ -714,9 +714,9 @@ for you transparently.
Caching and QuerySets
Each :class:`~django.db.models.query.QuerySet` contains a cache, to minimize
database access. It's important to understand how it works, in order to write
the most efficient code.
Each :class:`~django.db.models.query.QuerySet` contains a cache to minimize
database access. Understanding how it works will allow you to write the most
efficient code.
In a newly created :class:`~django.db.models.query.QuerySet`, the cache is
empty. The first time a :class:`~django.db.models.query.QuerySet` is evaluated
@@ -747,6 +747,43 @@ To avoid this problem, simply save the
>>> print([p.headline for p in queryset]) # Evaluate the query set.
>>> print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation.
When querysets are not cached
Querysets do not always cache their results. When evaluating only *part* of
the queryset, the cache is checked, but if it is not populated then the items
returned by the subsequent query are not cached. Specifically, this means that
:ref:`limiting the queryset <limiting-querysets>` using an array slice or an
index will not populate the cache.
For example, repeatedly getting a certain index in a queryset object will query
the database each time::
>>> queryset = Entry.objects.all()
>>> print queryset[5] # Queries the database
>>> print queryset[5] # Queries the database again
However, if the entire queryset has already been evaluated, the cache will be
checked instead::
>>> queryset = Entry.objects.all()
>>> [entry for entry in queryset] # Queries the database
>>> print queryset[5] # Uses cache
>>> print queryset[5] # Uses cache
Here are some examples of other actions that will result in the entire queryset
being evaluated and therefore populate the cache::
>>> [entry for entry in queryset]
>>> bool(queryset)
>>> entry in queryset
>>> list(queryset)
.. note::
Simply printing the queryset will not populate the cache. This is because
the call to ``__repr__()`` only returns a slice of the entire queryset.
.. _complex-lookups-with-q:
Complex lookups with Q objects

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