Automatically prefetching is something I feel should be avoided. A common gripe I have with ORMs is they hide what's actually happening with the database, resulting in beginners-going-on-intermediates building libraries/systems that don't scale well.
We have several views in a dashboard, where a relation may be accessed once or twice while iterating over a large python filtered queryset. Prefetching this relation based on the original queryset has the potential to add around 5 seconds to the response time (probably more, that table has doubled in size since I last measured it). I feel it would be better to optimise for your usecase, as apposed to try to prevent uncalled-for behaviour. On Aug 15, 2017 23:15, "Luke Plant" <l.plant...@cantab.net> wrote: > I agree with Marc here that the proposed optimizations are 'magical'. I > think when it comes to optimizations like these you simply cannot know in > advance whether doing extra queries is going to a be an optimization or a > pessimization. If I can come up with a single example where it would > significantly decrease performance (either memory usage or speed) compared > to the default (and I'm sure I can), then I would be strongly opposed to it > ever being default behaviour. > > Concerning implementing it as an additional QuerySet method like > `auto_prefetch()` - I'm not sure what I think, I feel like it could get > icky (i.e. increase our technical debt), due to the way it couples things > together. I can't imagine ever wanting to use it, though, I would always > prefer the manual option. > > Luke > > > > On 15/08/17 21:02, Marc Tamlyn wrote: > > Hi Gordon, > > Thanks for the suggestion. > > I'm not a fan of adding a layer that tries to be this clever. How would > possible prefetches be identified? What happens when an initial loop in a > view requires one prefetch, but a subsequent loop in a template requires > some other prefetch? What about nested loops resulting in nested > prefetches? Code like this is almost guaranteed to break unexpectedly in > multiple ways. Personally, I would argue that correctly setting up and > maintaining appropriate prefetches and selects is a necessary part of > working with an ORM. > > Do you know of any other ORMs which attempt similar magical optimisations? > How do they go about identifying the cases where it is necessary? > > On 15 August 2017 at 10:44, 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 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/ms >> gid/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. > 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/CAMwjO1Gaha-K7KkefJkiS3LRdXvaPPwBeuKmhQv6b > JFx3dty3w%40mail.gmail.com > <https://groups.google.com/d/msgid/django-developers/CAMwjO1Gaha-K7KkefJkiS3LRdXvaPPwBeuKmhQv6bJFx3dty3w%40mail.gmail.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. > 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/a5780df6-ce60-05ae-88e3-997e6bc88f5c%40cantab.net > <https://groups.google.com/d/msgid/django-developers/a5780df6-ce60-05ae-88e3-997e6bc88f5c%40cantab.net?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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