I'd like to discuss automatic prefetching in querysets. Specifically automatically doing prefetch_related where needed without the user having to request it.
For context consider these three snippets using the Question & Choice models from the tutorial <https://docs.djangoproject.com/en/1.11/intro/tutorial02/#creating-models> when there are 100 questions each with 5 choices for a total of 500 choices. Default for choice in Choice.objects.all(): print(choice.question.question_text, ':', choice.choice_text) 501 db queries, fetches 500 choice rows and 500 question rows from the DB Prefetch_related for choice in Choice.objects.prefetch_related('question'): print(choice.question.question_text, ':', choice.choice_text) 2 db queries, fetches 500 choice rows and 100 question rows from the DB Select_related for choice in Choice.objects.select_related('question'): print(choice.question.question_text, ':', choice.choice_text) 1 db query, fetches 500 choice rows and 500 question rows from the DB I've included select_related for completeness, I'm not going to propose changing anything about it's use. There are places where it is the best choice and in those places it will still be up to the user to request it. I will note that anywhere select_related is optimal prefetch_related is still better than the default and leave it at that. The 'Default' example above is a classic example of the N+1 query problem, a problem that is widespread in Django apps. This pattern of queries is what new users produce because they don't know enough about the database and / or ORM to do otherwise. Experieced users will also often produce this because it's not always obvious what fields will and won't be used and subsequently what should be prefetched. Additionally that list will change over time. A small change to a template to display an extra field can result in a denial of service on your DB due to a missing prefetch. Identifying missing prefetches is fiddly, time consuming and error prone. Tools like django-perf-rec <https://github.com/YPlan/django-perf-rec> (which I was involved in creating) and nplusone <https://github.com/jmcarp/nplusone> exist in part to flag missing prefetches introduced by changed code. Finally libraries like Django Rest Framework and the Admin will also produce queries like this because it's very difficult for them to know what needs prefetching without being explicitly told by an experienced user. As hinted at the top I'd like to propose changing Django so the default code behaves like the prefetch_related code. Longer term I think this should be the default behaviour but obviously it needs to be proved first so for now I'd suggest a new queryset function that enables this behaviour. I have a proof of concept of this mechanism that I've used successfully in production. I'm not posting it yet because I'd like to focus on desired behavior rather than implementation details. But in summary, what it does is when accessing a missing field on a model, rather than fetching it just for that instance, it runs a prefetch_related query to fetch it for all peer instances that were fetched in the same queryset. So in the example above it prefetches all Questions in one query. This might seem like a risky thing to do but I'd argue that it really isn't. The only time this isn't superior to the default case is when you are post filtering the queryset results in Python. Even in that case it's only inferior if you started with a large number of results, filtered basically all of them and the code is structured so that the filtered ones aren't garbage collected. To cover this rare case the automatic prefetching can easily be disabled on a per queryset or per object basis. Leaving us with a rare downside that can easily be manually resolved in exchange for a significant general improvement. In practice this thing is almost magical to work with. Unless you already have extensive and tightly maintained prefetches everywhere you get an immediate boost to virtually everything that touches the database, often knocking orders of magnitude off page load times. If an agreement can be reached on pursuing this then I'm happy to put in the work to productize the proof of concept. -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To post to this group, send email to django-developers@googlegroups.com. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/d402bf30-a5af-4072-8b50-85e921f7f9af%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
