This is right from the docs :
Caching and QuerySets¶ Each QuerySet contains a cache, to minimize database access. It's important to understand how it works, in order to write the most efficient code.
In a newly created QuerySet, the cache is empty. The first time a QuerySet is evaluated -- and, hence, a database query happens -- Django saves the query results in the QuerySet's cache and returns the results that have been explicitly requested (e.g., the next element, if the QuerySet is being iterated over). Subsequent evaluations of the QuerySet reuse the cached results.
Keep this caching behavior in mind, because it may bite you if you don't use your QuerySets correctly. For example, the following will create two QuerySets, evaluate them, and throw them away:
>>> print([e.headline for e in Entry.objects.all()])
>>> print([e.pub_date for e in Entry.objects.all()])
That means the same database query will be executed twice, effectively doubling your database load. Also, there's a possibility the two lists may not include the same database records, because an Entry may have been added or deleted in the split second between the two requests.
To avoid this problem, simply save the QuerySet and reuse it:
>>> queryset = Entry.objects.all()
>>> 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.
Also See These :-
https://docs.djangoproject.com/en/dev/topics/db/optimization/#understand-queryset-evaluation
http://stackoverflow.com/questions/4631865/caching-query-results-in-django