This received some positive responses, so to help move the conversation along I have created a ticket and pull request.
https://code.djangoproject.com/ticket/28586 https://github.com/django/django/pull/9064 Regards G On Tue, Aug 15, 2017 at 10:44 AM, Gordon Wrigley <gordon.wrig...@gmail.com> wrote: > 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 a topic in the > Google Groups "Django developers (Contributions to Django itself)" group. > To unsubscribe from this topic, visit https://groups.google.com/d/ > topic/django-developers/EplZGj-ejvg/unsubscribe. > To unsubscribe from this group and all its topics, 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 > <https://groups.google.com/d/msgid/django-developers/d402bf30-a5af-4072-8b50-85e921f7f9af%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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