Why is iterating through a large Django QuerySet consuming massive amounts of memory?

Question

Nate C was close, but not quite.

From the docs:

You can evaluate a QuerySet in the following ways:

• Iteration. A QuerySet is iterable, and it executes its database query the first time you iterate over it. For example, this will print the headline of all entries in the database:

  for e in Entry.objects.all():
      print e.headline
  

So your ten million rows are retrieved, all at once, when you first enter that loop and get the iterating form of the queryset. The wait you experience is Django loading the database rows and creating objects for each one, before returning something you can actually iterate over. Then you have everything in memory, and the results come spilling out.

From my reading of the docs, iterator() does nothing more than bypass QuerySet's internal caching mechanisms. I think it might make sense for it to a do a one-by-one thing, but that would conversely require ten-million individual hits on your database. Maybe not all that desirable.

Iterating over large datasets efficiently is something we still haven't gotten quite right, but there are some snippets out there you might find useful for your purposes:

• Memory Efficient Django QuerySet iterator
• batch querysets
• QuerySet Foreach

Might not be the faster or most efficient, but as a ready-made solution why not use django core's Paginator and Page objects documented here:

https://docs.djangoproject.com/en/dev/topics/pagination/

Something like this:

from django.core.paginator import Paginator
from djangoapp.models import model

paginator = Paginator(model.objects.all(), 1000) # chunks of 1000, you can 
                                                 # change this to desired chunk size

for page in range(1, paginator.num_pages + 1):
    for row in paginator.page(page).object_list:
        # here you can do whatever you want with the row
    print "done processing page %s" % page

Django's default behavior is to cache the whole result of the QuerySet when it evaluates the query. You can use the QuerySet's iterator method to avoid this caching:

for event in Event.objects.all().iterator():
    print event

https://docs.djangoproject.com/en/dev/ref/models/querysets/#iterator

The iterator() method evaluates the queryset and then reads the results directly without doing caching at the QuerySet level. This method results in better performance and a significant reduction in memory when iterating over a large number of objects that you only need to access once. Note that caching is still done at the database level.

Using iterator() reduces memory usage for me, but it is still higher than I expected. Using the paginator approach suggested by mpaf uses much less memory, but is 2-3x slower for my test case.

from django.core.paginator import Paginator

def chunked_iterator(queryset, chunk_size=10000):
    paginator = Paginator(queryset, chunk_size)
    for page in range(1, paginator.num_pages + 1):
        for obj in paginator.page(page).object_list:
            yield obj

for event in chunked_iterator(Event.objects.all()):
    print event

This is from the docs: http://docs.djangoproject.com/en/dev/ref/models/querysets/

No database activity actually occurs until you do something to evaluate the queryset.

So when the print event is run the query fires (which is a full table scan according to your command.) and loads the results. Your asking for all the objects and there is no way to get the first object without getting all of them.

But if you do something like:

Event.objects.all()[300:900]

http://docs.djangoproject.com/en/dev/topics/db/queries/#limiting-querysets

Then it will add offsets and limits to the sql internally.

For large amounts of records, a database cursor performs even better. You do need raw SQL in Django, the Django-cursor is something different than a SQL cursur.

The LIMIT - OFFSET method suggested by Nate C might be good enough for your situation. For large amounts of data it is slower than a cursor because it has to run the same query over and over again and has to jump over more and more results.

Django doesn't have good solution for fetching large items from database.

import gc
# Get the events in reverse order
eids = Event.objects.order_by("-id").values_list("id", flat=True)

for index, eid in enumerate(eids):
    event = Event.object.get(id=eid)
    # do necessary work with event
    if index % 100 == 0:
       gc.collect()
       print("completed 100 items")

values_list can be used to fetch all the ids in the databases and then fetch each object separately. Over a time large objects will be created in memory and won't be garbage collected til for loop is exited. Above code does manual garbage collection after every 100th item is consumed.