I believe we should be optimising for the **common** use case, not 
expecting everyone to be experts with the ORM.

> 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. 

The status quo is already one where thousands of users and sites are doing 
the non-optimal thing because we're choosing to be conservative and have 
users opt-in to the optimal behaviour. A massive complaint against Django 
is how easy it is for users to build in 1+N behaviour. Django is supposed 
to abstract the database away (so much so that we avoid SQL related terms 
in our queryset methods), yet one of the more fundamental concepts such as 
joins we expect users to know about and optimise for.

I'd be more in favour of throwing an error on 
non-select-related-foreign-key-access than what we're currently doing which 
is a query for each access.

The only options users currently have of monitoring poor behaviour is:

1. Add logging to django.db.models
2. Add django-debug-toolbar
3. Investigate page slow downs

Here's a bunch of ways that previously tuned queries can "go bad":

1. A models `__str__` method is updated to include a related field
2. A template uses a previously unused related field
3. A report uses a previously unused related field
4. A ModelAdmin adds a previously unused related field

I think a better question to ask is:

- How many people have had their day/site ruined because we think 
auto-prefetching is too magical?
- How many people would have their day/site ruined because we think 
auto-prefetching is the better default?

If we were introducing a new ORM, I think the above answer would be obvious 
given what we know of Django use today.

What I'd propose:

1. (optional) A global setting to disable autoprefetching
2. An opt out per queryset
3. (optional) An opt out per Meta?
4. Logging any autoprefetches - perhaps as a warning?

More experienced Django users that do not want this behaviour are going to 
know about a global setting and can opt in to the old behaviour rather 
easily. Newer users that do not know about select/prefetch_related or these 
settings will fall into the new behaviour by default.

It's unreasonable to expect every user of django learn the ins and outs of 
all queryset methods. I'm probably considered a django orm expert, and I 
still sometimes write queries that are non-optimal or *become* non-optimal 
after changes in unrelated areas. At an absolute minimum we should be 
screaming and shouting when this happens. But we can also fix the issue 
while complaining, and help guide users into correct behaviour.


On Wednesday, 16 August 2017 08:41:31 UTC+10, Anthony King wrote:
>
> 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.pla...@cantab.net <javascript:>> 
> 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....@gmail.com 
>> <javascript:>> 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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