Chris W. Chris W. - 1 year ago 104
Python Question

Limiting Memory Use in a *Large* Django QuerySet

I have a task which needs to be run on 'most' objects in my database once every some period of time (once a day, once a week, whatever). Basically this means that I have some query that looks like this running in it's own thread.

for model_instance in SomeModel.objects.all():

(Note that it's actually a filter() not all() but none-the-less I still end up selecting a very large set of objects.)

The problem I'm running into is that after running for a while the thread is killed by my hosting provider because I'm using too much memory. I'm assuming all this memory use is happening because even though the
object returned by my query initially has a very small memory footprint it ends up growing as the
object caches each
as I iterate through them.

My question is, "what is the best way to iterate through almost every
in my database in a memory efficient way?" or perhaps my question is "how do I 'un-cache' model instances from a django queryset?"

EDIT: I'm actually using the results of the queryset to build a series of new objects. As such, I don't end up updating the queried-for objects at all.

Answer Source

So what I actually ended up doing is building something that you can 'wrap' a QuerySet in. It works by making a deepcopy of the QuerySet, using the slice syntax--e.g., some_queryset[15:45]--but then it makes another deepcopy of the original QuerySet when the slice has been completely iterated through. This means that only the set of Objects returned in 'this' particular slice are stored in memory.

class MemorySavingQuerysetIterator(object):

    def __init__(self,queryset,max_obj_num=1000):
        self._base_queryset = queryset
        self._generator = self._setup()
        self.max_obj_num = max_obj_num

    def _setup(self):
        for i in xrange(0,self._base_queryset.count(),self.max_obj_num):
            # By making a copy of of the queryset and using that to actually access
            # the objects we ensure that there are only `max_obj_num` objects in
            # memory at any given time
            smaller_queryset = copy.deepcopy(self._base_queryset)[i:i+self.max_obj_num]
            logger.debug('Grabbing next %s objects from DB' % self.max_obj_num)
            for obj in smaller_queryset.iterator():
                yield obj

    def __iter__(self):
        return self

    def next(self):

So instead of...

for obj in SomeObject.objects.filter(foo='bar'): <-- Something that returns *a lot* of Objects

You would do...

for obj in MemorySavingQuerysetIterator(in SomeObject.objects.filter(foo='bar')):

Please note that the intention of this is to save memory in your Python interpreter. It essentially does this by making more database queries. Usually people are trying to do the exact opposite of that--i.e., minimize database queries as much as possible without regards to memory usage. Hopefully somebody will find this useful though.

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