Count vs len on a Django QuerySet

Question

In Django, given that I have a QuerySet that I am going to iterate over and print the results of, what is the best option for counting the objects? len(qs

Answer 1

Choosing between len() and count() depends on the situation and it's worth to deeply understand how they work to use them correctly.

Let me provide you with few scenarios:

1. (most crucial) When you only want to know the number of elements and you do not plan to process them in any way it's crucial to use count():

   DO: queryset.count() - this will perform single SELECT COUNT(*) some_table query, all computation is carried on RDBMS side, Python just needs to retrieve the result number with fixed cost of O(1)

   DON'T: len(queryset) - this will perform SELECT * FROM some_table query, fetching whole table O(N) and requiring additional O(N) memory for storing it. This is the worst that can be done

2. When you intend to fetch the queryset anyway it's slightly better to use len() which won't cause an extra database query as count() would:

   len(queryset) # fetching all the data - NO extra cost - data would be fetched anyway in the for loop
   
   for obj in queryset: # data is already fetched by len() - using cache
       pass
   

   Count:

   queryset.count() # this will perform an extra db query - len() did not
   
   for obj in queryset: # fetching data
       pass
   

3. Reverted 2nd case (when queryset has already been fetched):

   for obj in queryset: # iteration fetches the data
       len(queryset) # using already cached data - O(1) no extra cost
       queryset.count() # using cache - O(1) no extra db query
   
   len(queryset) # the same O(1)
   queryset.count() # the same: no query, O(1)
   

Everything will be clear once you take a glance "under the hood":

class QuerySet(object):

    def __init__(self, model=None, query=None, using=None, hints=None):
        # (...)
        self._result_cache = None

    def __len__(self):
        self._fetch_all()
        return len(self._result_cache)

    def _fetch_all(self):
        if self._result_cache is None:
            self._result_cache = list(self.iterator())
        if self._prefetch_related_lookups and not self._prefetch_done:
            self._prefetch_related_objects()

    def count(self):
        if self._result_cache is not None:
            return len(self._result_cache)

        return self.query.get_count(using=self.db)

Good references in Django docs:

• When QuerySets are evaluated
• count()