def queryset_iterator(queryset, chunksize=1000):
    """
    Iterate over a Django Queryset ordered by the primary key.

    This method loads a maximum of chunksize (default: 1000) rows in it's
    memory at the same time while django normally would load all rows in it's
    memory.

    Note that this iterator does not support ordered query sets.

    from http://www.mellowmorning.com/2010/03/03/django-query-set-iterator-for-really-large-querysets/
    with some local optimizations.
    """

    last_pk = queryset.order_by('-pk')[0].pk
    queryset = queryset.order_by('pk')
    pk = queryset[0].pk
    while pk < last_pk:
        for row in queryset.filter(pk__gte=pk, pk__lte=last_pk)[:chunksize]:
            yield row
        pk += chunksize