Sorry maybe I wasn't clear enough about the proposed mechanism.

Currently when you dereference a foreign key field on an object (so
'choice.question' in the examples above) if it doesn't have the value
cached from an earlier access, prefetch_related or select_related then
Django will automatically perform a db query to fetch it. After that the
value will then be cached on the object for any future dereferences.

This automatic fetching is the source the N+1 query problems and in my
experience most gross performance problems in Django apps.

The proposal essentially is to add a new queryset function that says for
the group of objects fetched by this queryset, whenever one of these
automatic foreign key queries happens on one of them instead of fetching
the foreign key for just that one use the prefetch mechanism to fetch it
for all of them.
The presumption being that the vast majority of the time when you access a
field on one object from a queryset result, probably you are going to
access the same field on many of the others as well.

The implementation I've used in production does nest across foreign keys so
something (admittedly contrived) like:
for choice in Choice.objects.all():
    print(choice.question.author)
Will produce 3 queries, one for all choices, one for the questions of those
choices and one for the authors of those questions.

It's worth noting that because these are foreign keys in their "to one"
direction each of those queryset results will be at most the same size (in
rows) as the proceeding one and often (due to nulls and duplicates) smaller.

I do not propose touching reverse foreign key or many2many fields as the
generated queries could request substantially more rows from the DB than
the original query and it's not at all clear how this mechanism would
sanely interact with filtering etc. So this is purely about the forward
direction of foreign keys.

I hope that clarifies my thinking some.

Regards
G

On Tue, Aug 15, 2017 at 7:02 PM, Marc Tamlyn <marc.tam...@gmail.com> 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.
>>
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