Re: Automatic prefetching in querysets

2017-09-12 Thread Gordon Wrigley
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 
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
>  
> 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 
> (which I was involved in creating) and 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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Re: Automatic prefetching in querysets

2017-08-30 Thread Patryk Zawadzki
W dniu środa, 16 sierpnia 2017 14:48:54 UTC+2 użytkownik Luke Plant napisał:
>
> I completely agree that visibility of this problem is a major issue, and 
> would really welcome work on improving this, especially in DEBUG mode. One 
> option might be a method that replaces lazy loads with exceptions. This 
> would force you to add the required `prefetch_related` or `select_related` 
> calls. You could have:
>
> .lazy_load(None)   # exception on accessing any non-loaded FK objects
>
> .lazy_load('my_fk1', 'my_fk2')  # exceptions on accessing any unloaded FK 
> objects apart from the named ones
>
> .lazy_load('__any__')   # cancel the above, restore default behaviour
>
> This (or something like it) would be a different way to tackle the problem 
> - just throwing it out. You could have a Meta option to do 
> `.lazy_load(None)` by default for a Model.  I've no idea how practical it 
> would be to wire this all up so that is works correctly, and with nested 
> objects etc.
>

Would love if Django could be told to require explicit data fetches. This 
and replacing QuerySet.__iter__ with an explicit fetch() (and lazy_fetch()) 
would make it much easier to reason about large codebases.

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Re: Automatic prefetching in querysets

2017-08-21 Thread Shai Berger
On Monday 21 August 2017 18:44:35 Tobias McNulty wrote:
> On Sat, Aug 19, 2017 at 1:10 PM, Luke Plant  wrote:
> > This could work something like the way that ForeignKey `on_delete` works
> > - you have options that are enumerated as constants, but in reality they
> > are classes that embody the strategy to be used. We could have something
> > similar - `on_missing_relation=FETCH | WARN | ERROR | IGNORE ... `
> 
> I like this a lot. It (1) caters (I think) to many if not all of the
> behavior preferences expressed in this thread, (2) mimics an existing API
> we support, (3) allows projects to gradually add/try the feature, and (4)
> permits but does not require a deprecation path for changing the default
> behavior in the future. I'm assuming `FETCH` represents the current
> behavior and not auto-detection of a missing relation and modification of
> the associated QuerySet.
> 

I agree that the idea sounds nice, with one exception: It seems like this 
would make sense, mostly, at the foreign-key-definition level, which means the 
feature is applied at model scope -- altough it isn't quite clear. However, 
the main argument so far was exactly about the right scope to use, with four 
different suggestions as far as I could see (queryset, model, context-manager 
and a global setting), each with its own pros and cons. So, it feels a little 
like going off the main thread of discussion.

> One alternative thought: Could we define two `ForeignKey` options (again
> using the `on_delete` analogy) which support adding the relation to
> select_related()/prefetch_related() all the time (e.g.,
> `ForeignKey(prefetch_related=ALWAYS | MANUALLY | WARN | ERROR)` and/or
> `ForeignKey(select_related=ALWAYS | MANUALLY | WARN | ERROR)`, where
> MANUALLY represents the current behavior)? One could then use `.only()` or
> `.values*()` to avoid fetching the related model, if needed.
> 

This is truly, clearly off-topic here; I just want to point out that the effect 
can already be achieved by overriding the model's default manager's 
get_queryset(). The idea might have merits in terms of maintainability and 
readability, but I'd much rather it be discussed separately.

Shai


Re: Automatic prefetching in querysets

2017-08-21 Thread Tobias McNulty
On Sat, Aug 19, 2017 at 1:10 PM, Luke Plant  wrote:

> This could work something like the way that ForeignKey `on_delete` works -
> you have options that are enumerated as constants, but in reality they are
> classes that embody the strategy to be used. We could have something
> similar - `on_missing_relation=FETCH | WARN | ERROR | IGNORE ... `
>
I like this a lot. It (1) caters (I think) to many if not all of the
behavior preferences expressed in this thread, (2) mimics an existing API
we support, (3) allows projects to gradually add/try the feature, and (4)
permits but does not require a deprecation path for changing the default
behavior in the future. I'm assuming `FETCH` represents the current
behavior and not auto-detection of a missing relation and modification of
the associated QuerySet.

One alternative thought: Could we define two `ForeignKey` options (again
using the `on_delete` analogy) which support adding the relation to
select_related()/prefetch_related() all the time (e.g.,
`ForeignKey(prefetch_related=ALWAYS | MANUALLY | WARN | ERROR)` and/or
`ForeignKey(select_related=ALWAYS | MANUALLY | WARN | ERROR)`, where
MANUALLY represents the current behavior)? One could then use `.only()` or
`.values*()` to avoid fetching the related model, if needed.


*Tobias McNulty*Chief Executive Officer

tob...@caktusgroup.com
www.caktusgroup.com

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Re: Automatic prefetching in querysets

2017-08-19 Thread Luke Plant
This could work something like the way that ForeignKey `on_delete` works 
- you have options that are enumerated as constants, but in reality they 
are classes that embody the strategy to be used. We could have something 
similar - `on_missing_relation=FETCH | WARN | ERROR | IGNORE ... `


Luke

On 18/08/17 14:01, Shai Berger wrote:

On Friday 18 August 2017 09:08:11 Anssi Kääriäinen wrote:

Maybe we should just add the queryset method. This is the smallest atomic
task that can be done. Even if there's only the queryset method available,
it's possible to enable prefetches per model by using a Manager.


I disagree on both counts: I don't think it's the smallest atomic task, and
I'm not so sure it's the right thing to do.

The smallest atomic task, the way I see it, is building the infrastructure
that would allow adding the queryset method -- but would also allow different
APIs to be set around it.

And since there is as yet no consensus on the correct API for "end" users, I
would rather not define one immediately.

Shai.



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Re: Automatic prefetching in querysets

2017-08-19 Thread Todor Velichkov
+1 to just add the queryset method it gives a fine granular level of 
control. Moreover when not used we could display warnings which can help 
people detect these O(n) query cases as Tom Forbes initially suggested.

On Friday, August 18, 2017 at 9:08:11 AM UTC+3, Anssi Kääriäinen wrote:
>
> On Thursday, August 17, 2017 at 11:30:45 PM UTC+3, Aymeric Augustin wrote:
>>
>> Hello, 
>>
>> > On 17 Aug 2017, at 14:48, Luke Plant  wrote: 
>> > 
>> > On 16/08/17 23:17, Aymeric Augustin wrote: 
>> >> It should kick in only when no select_related or prefetch_related is 
>> in effect, to avoid interfering with pre-existing optimizations. It's still 
>> easy to construct an example where it would degrade performance but I don't 
>> think such situations will be common in practice. 
>> > 
>> > I think Ansii's example is a non-contrived and potentially very common 
>> one where we will make things worse if this is default behaviour, 
>> especially for the non-expert programmer this behaviour is supposed to 
>> help: 
>> > 
>> > 
>> > if qs: 
>> > qs[0].do_something() 
>> > 
>> > (where do_something() access relations, or multiple relations). We will 
>> end up with *multiple* large unnecessary queries instead of just one. The 
>> difference is that the first one is easy to explain, the remainder are much 
>> harder, and will contribute even more to the "Django is slow/ORMs are bad" 
>> feeling. 
>>
>>
>> I wasn't convinced by this example because it's already sub-optimal 
>> currently. `if qs` will fetch the whole queryset just to check if it has at 
>> least one element. 
>>
>> Assuming there's 1000 elements in qs, if you don't care about a 1000x 
>> inefficiency, I don't expect you to care much about two or three 1000x 
>> inefficiencies...
>>
>
> I agree that this example isn't particularly worrying. It's something an 
> experienced developer wouldn't do. On the other hand, we are aiming at 
> making things simpler for non-experienced developers.
>
> To me the worrying part here is that we really don't have any data or 
> experience about if the cure will be worse than the disease. Likely not, 
> but what do we gain by taking risks here?
>
> Maybe we should just add the queryset method. This is the smallest atomic 
> task that can be done. Even if there's only the queryset method available, 
> it's possible to enable prefetches per model by using a Manager.
>
>  - Anssi
>
>
>

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Re: Automatic prefetching in querysets

2017-08-18 Thread Collin Anderson
I like that idea - keep it a private API for now and make it a public API
once people have used it a little bit.

On Fri, Aug 18, 2017 at 4:01 AM, Shai Berger  wrote:

> On Friday 18 August 2017 09:08:11 Anssi Kääriäinen wrote:
> > Maybe we should just add the queryset method. This is the smallest atomic
> > task that can be done. Even if there's only the queryset method
> available,
> > it's possible to enable prefetches per model by using a Manager.
> >
> I disagree on both counts: I don't think it's the smallest atomic task, and
> I'm not so sure it's the right thing to do.
>
> The smallest atomic task, the way I see it, is building the infrastructure
> that would allow adding the queryset method -- but would also allow
> different
> APIs to be set around it.
>
> And since there is as yet no consensus on the correct API for "end" users,
> I
> would rather not define one immediately.
>
> Shai.
>

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Re: Automatic prefetching in querysets

2017-08-18 Thread Shai Berger
On Friday 18 August 2017 09:08:11 Anssi Kääriäinen wrote:
> Maybe we should just add the queryset method. This is the smallest atomic
> task that can be done. Even if there's only the queryset method available,
> it's possible to enable prefetches per model by using a Manager.
> 
I disagree on both counts: I don't think it's the smallest atomic task, and 
I'm not so sure it's the right thing to do.

The smallest atomic task, the way I see it, is building the infrastructure 
that would allow adding the queryset method -- but would also allow different 
APIs to be set around it.

And since there is as yet no consensus on the correct API for "end" users, I 
would rather not define one immediately.

Shai.


Re: Automatic prefetching in querysets

2017-08-18 Thread Anssi Kääriäinen
On Thursday, August 17, 2017 at 11:30:45 PM UTC+3, Aymeric Augustin wrote:
>
> Hello, 
>
> > On 17 Aug 2017, at 14:48, Luke Plant  
> wrote: 
> > 
> > On 16/08/17 23:17, Aymeric Augustin wrote: 
> >> It should kick in only when no select_related or prefetch_related is in 
> effect, to avoid interfering with pre-existing optimizations. It's still 
> easy to construct an example where it would degrade performance but I don't 
> think such situations will be common in practice. 
> > 
> > I think Ansii's example is a non-contrived and potentially very common 
> one where we will make things worse if this is default behaviour, 
> especially for the non-expert programmer this behaviour is supposed to 
> help: 
> > 
> > 
> > if qs: 
> > qs[0].do_something() 
> > 
> > (where do_something() access relations, or multiple relations). We will 
> end up with *multiple* large unnecessary queries instead of just one. The 
> difference is that the first one is easy to explain, the remainder are much 
> harder, and will contribute even more to the "Django is slow/ORMs are bad" 
> feeling. 
>
>
> I wasn't convinced by this example because it's already sub-optimal 
> currently. `if qs` will fetch the whole queryset just to check if it has at 
> least one element. 
>
> Assuming there's 1000 elements in qs, if you don't care about a 1000x 
> inefficiency, I don't expect you to care much about two or three 1000x 
> inefficiencies...
>

I agree that this example isn't particularly worrying. It's something an 
experienced developer wouldn't do. On the other hand, we are aiming at 
making things simpler for non-experienced developers.

To me the worrying part here is that we really don't have any data or 
experience about if the cure will be worse than the disease. Likely not, 
but what do we gain by taking risks here?

Maybe we should just add the queryset method. This is the smallest atomic 
task that can be done. Even if there's only the queryset method available, 
it's possible to enable prefetches per model by using a Manager.

 - Anssi


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Re: Automatic prefetching in querysets

2017-08-17 Thread Aymeric Augustin
Hello,

> On 17 Aug 2017, at 14:48, Luke Plant  wrote:
> 
> On 16/08/17 23:17, Aymeric Augustin wrote:
>> It should kick in only when no select_related or prefetch_related is in 
>> effect, to avoid interfering with pre-existing optimizations. It's still 
>> easy to construct an example where it would degrade performance but I don't 
>> think such situations will be common in practice.
> 
> I think Ansii's example is a non-contrived and potentially very common one 
> where we will make things worse if this is default behaviour, especially for 
> the non-expert programmer this behaviour is supposed to help:
> 
> 
> if qs:
> qs[0].do_something()
> 
> (where do_something() access relations, or multiple relations). We will end 
> up with *multiple* large unnecessary queries instead of just one. The 
> difference is that the first one is easy to explain, the remainder are much 
> harder, and will contribute even more to the "Django is slow/ORMs are bad" 
> feeling.


I wasn't convinced by this example because it's already sub-optimal currently. 
`if qs` will fetch the whole queryset just to check if it has at least one 
element.

Assuming there's 1000 elements in qs, if you don't care about a 1000x 
inefficiency, I don't expect you to care much about two or three 1000x 
inefficiencies...

-- 
Aymeric.


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Re: Automatic prefetching in querysets

2017-08-17 Thread Collin Anderson
"turn it on/off per model" - I wonder if we just have a custom
manager/mixin for the per model case. objects =
AutoPrefetchRelatedManager(). The manager can return a queryset with
prefetch_related(auto=True) or whatever already applied.

On Thu, Aug 17, 2017 at 1:33 PM, Andrew Godwin  wrote:

> Just some quick thoughts, as most of the points I would bring up have been
> discussed:
>
> - This should definitely be opt-in for the first release, it's too big a
> change to make opt-out straight away.
>
> - You should be able to turn it on/off per model, probably in Meta, not in
> a setting, and obviously be able to override it on the queryset level
>
> - I am concerned for the nasty performance edge cases this will cause with
> single-object access patterns (like Anssi's example), but I think the
> overall gain would likely be worth it.
>
> In general, very large sites won't be able to use this at all as JOINs or
> cross-table queries in one piece of code aren't allowed, but they probably
> already have plenty of expertise in this area. We also need to give
> consideration to how it will interact with multiple database support, and
> third-party solutions to things like sharding.
>
> Andrew
>
> On Thu, Aug 17, 2017 at 2:02 AM, Malcolm Box  wrote:
>
>> Hi,
>>
>> I think there's a potential to make this opt-in, and improve the
>> out-of-box experience.
>>
>> Summarising the discussion, it seems that the rough consensus is that if
>> we were building the ORM from scratch, then this would be entirely sensible
>> behaviour (with necessary per-QS ways to disable) - it would remove a
>> common performance problem (N+1 queries), would improve areas where adding
>> prefetch_related to queries is awkward, and in rare cases where it
>> decreased performance there would be documented ways to fix.
>>
>> So the main disagreement is about how to get there from here, and there's
>> concern about three types of users:
>>
>> 1 - Existing, non-expert users whose sites work now (because otherwise
>> they would have already fixed the problem) but who have lurking N+1 issues
>> which will break them later
>> 2 - Existing, non-expert users whose sites work now, but would have
>> performance issues if this was enabled by an upgrade
>> 3 - Existing, expert users who have already found and fixed the issues
>> and who could therefore get no benefit but might suffer a performance
>> degradation.
>>
>> I'll assert that the size of these populations above is listed in roughly
>> size order, with #1 being the biggest. This is a hunch based on most sites
>> not having huge tables where N+1 becomes a problem - at least not until
>> they've been running for a few years and accumulated lots of data...
>>
>> There is another population that hasn't been considered - users starting
>> django projects (ie people running django-admin startproject). Over time,
>> this is by far the largest population.
>>
>> So would a sensible approach be:
>>
>> - Feature is globally opt-in
>> - startproject opts in for new projects
>> - Release notes mention the new flag loudly, and encourage people to try
>> switching it on
>> - We add the debug tracing to help people find places where this setting
>> would help - and encourage them to enable it globally before trying
>> individual queryset.prefetch_related
>>
>> Then over time, all new projects will have the new behaviour. Old
>> projects will gradually upgrade - everyone in category 1 will hit the "make
>> it work" switch the first time they see the warning / see a problem.
>> Experts can choose how they migrate - as Adam points out, even experts can
>> miss things.
>>
>> Finally after a suitable warning period, this can become an opt-out
>> feature and we arrive in the sunny world of an ORM that works better for
>> all users.
>>
>> Cheers,
>>
>> Malcolm
>>
>> On Wednesday, 16 August 2017 21:17:49 UTC+1, Aymeric Augustin wrote:
>>>
>>> On 15 Aug 2017, at 11:44, Gordon Wrigley  wrote:
>>>
>>> I'd like to discuss automatic prefetching in querysets. Specifically
>>> automatically doing prefetch_related where needed without the user having
>>> to request it.
>>>
>>>
>>>
>>> Hello,
>>>
>>> I'm rather sympathetic to this proposal. Figuring out N + 1 problems in
>>> the admin or elsewhere gets old.
>>>
>>>
>>> In addition to everything that was said already, I'd like to point out
>>> that Django already has a very similar "magic auto prefetching" behavior in
>>> some cases :-)
>>>
>>> I'm referring to the admin which calls select_related() on non-nullable
>>> foreign keys in the changelist view. The "non-nullable" condition makes
>>> that behavior hard to predict — I'd go as far as to call it non
>>> deterministic. For details, see slide 54 of https://myks.org/data/20161
>>> 103-Django_Under_the_Hood-Debugging_Performance.pdf and the audio
>>> commentary at https://youtu.be/5fheDDj3oHY?t=2024.
>>>
>>>
>>> The feature proposed here is most useful if it's 

Re: Automatic prefetching in querysets

2017-08-17 Thread Andrew Godwin
Just some quick thoughts, as most of the points I would bring up have been
discussed:

- This should definitely be opt-in for the first release, it's too big a
change to make opt-out straight away.

- You should be able to turn it on/off per model, probably in Meta, not in
a setting, and obviously be able to override it on the queryset level

- I am concerned for the nasty performance edge cases this will cause with
single-object access patterns (like Anssi's example), but I think the
overall gain would likely be worth it.

In general, very large sites won't be able to use this at all as JOINs or
cross-table queries in one piece of code aren't allowed, but they probably
already have plenty of expertise in this area. We also need to give
consideration to how it will interact with multiple database support, and
third-party solutions to things like sharding.

Andrew

On Thu, Aug 17, 2017 at 2:02 AM, Malcolm Box  wrote:

> Hi,
>
> I think there's a potential to make this opt-in, and improve the
> out-of-box experience.
>
> Summarising the discussion, it seems that the rough consensus is that if
> we were building the ORM from scratch, then this would be entirely sensible
> behaviour (with necessary per-QS ways to disable) - it would remove a
> common performance problem (N+1 queries), would improve areas where adding
> prefetch_related to queries is awkward, and in rare cases where it
> decreased performance there would be documented ways to fix.
>
> So the main disagreement is about how to get there from here, and there's
> concern about three types of users:
>
> 1 - Existing, non-expert users whose sites work now (because otherwise
> they would have already fixed the problem) but who have lurking N+1 issues
> which will break them later
> 2 - Existing, non-expert users whose sites work now, but would have
> performance issues if this was enabled by an upgrade
> 3 - Existing, expert users who have already found and fixed the issues and
> who could therefore get no benefit but might suffer a performance
> degradation.
>
> I'll assert that the size of these populations above is listed in roughly
> size order, with #1 being the biggest. This is a hunch based on most sites
> not having huge tables where N+1 becomes a problem - at least not until
> they've been running for a few years and accumulated lots of data...
>
> There is another population that hasn't been considered - users starting
> django projects (ie people running django-admin startproject). Over time,
> this is by far the largest population.
>
> So would a sensible approach be:
>
> - Feature is globally opt-in
> - startproject opts in for new projects
> - Release notes mention the new flag loudly, and encourage people to try
> switching it on
> - We add the debug tracing to help people find places where this setting
> would help - and encourage them to enable it globally before trying
> individual queryset.prefetch_related
>
> Then over time, all new projects will have the new behaviour. Old projects
> will gradually upgrade - everyone in category 1 will hit the "make it work"
> switch the first time they see the warning / see a problem. Experts can
> choose how they migrate - as Adam points out, even experts can miss things.
>
> Finally after a suitable warning period, this can become an opt-out
> feature and we arrive in the sunny world of an ORM that works better for
> all users.
>
> Cheers,
>
> Malcolm
>
> On Wednesday, 16 August 2017 21:17:49 UTC+1, Aymeric Augustin wrote:
>>
>> On 15 Aug 2017, at 11:44, Gordon Wrigley  wrote:
>>
>> I'd like to discuss automatic prefetching in querysets. Specifically
>> automatically doing prefetch_related where needed without the user having
>> to request it.
>>
>>
>>
>> Hello,
>>
>> I'm rather sympathetic to this proposal. Figuring out N + 1 problems in
>> the admin or elsewhere gets old.
>>
>>
>> In addition to everything that was said already, I'd like to point out
>> that Django already has a very similar "magic auto prefetching" behavior in
>> some cases :-)
>>
>> I'm referring to the admin which calls select_related() on non-nullable
>> foreign keys in the changelist view. The "non-nullable" condition makes
>> that behavior hard to predict — I'd go as far as to call it non
>> deterministic. For details, see slide 54 of https://myks.org/data/20161
>> 103-Django_Under_the_Hood-Debugging_Performance.pdf and the audio
>> commentary at https://youtu.be/5fheDDj3oHY?t=2024.
>>
>>
>> The feature proposed here is most useful if it's opt-out because it
>> targets people who aren't aware that the problem even exists — at best they
>> notice that Django is slow and that reminds them vaguely of a rant that
>> explains why ORMs are the worst thing since object oriented programming.
>>
>> It should kick in only when no select_related or prefetch_related is in
>> effect, to avoid interfering with pre-existing optimizations. It's still
>> easy to construct an example where it 

Re: Automatic prefetching in querysets

2017-08-17 Thread Malcolm Box
Hi,

I think there's a potential to make this opt-in, and improve the out-of-box 
experience.

Summarising the discussion, it seems that the rough consensus is that if we 
were building the ORM from scratch, then this would be entirely sensible 
behaviour (with necessary per-QS ways to disable) - it would remove a 
common performance problem (N+1 queries), would improve areas where adding 
prefetch_related to queries is awkward, and in rare cases where it 
decreased performance there would be documented ways to fix.

So the main disagreement is about how to get there from here, and there's 
concern about three types of users:

1 - Existing, non-expert users whose sites work now (because otherwise they 
would have already fixed the problem) but who have lurking N+1 issues which 
will break them later
2 - Existing, non-expert users whose sites work now, but would have 
performance issues if this was enabled by an upgrade
3 - Existing, expert users who have already found and fixed the issues and 
who could therefore get no benefit but might suffer a performance 
degradation.

I'll assert that the size of these populations above is listed in roughly 
size order, with #1 being the biggest. This is a hunch based on most sites 
not having huge tables where N+1 becomes a problem - at least not until 
they've been running for a few years and accumulated lots of data...

There is another population that hasn't been considered - users starting 
django projects (ie people running django-admin startproject). Over time, 
this is by far the largest population. 

So would a sensible approach be:

- Feature is globally opt-in 
- startproject opts in for new projects
- Release notes mention the new flag loudly, and encourage people to try 
switching it on
- We add the debug tracing to help people find places where this setting 
would help - and encourage them to enable it globally before trying 
individual queryset.prefetch_related

Then over time, all new projects will have the new behaviour. Old projects 
will gradually upgrade - everyone in category 1 will hit the "make it work" 
switch the first time they see the warning / see a problem. Experts can 
choose how they migrate - as Adam points out, even experts can miss things.

Finally after a suitable warning period, this can become an opt-out feature 
and we arrive in the sunny world of an ORM that works better for all users.

Cheers,

Malcolm

On Wednesday, 16 August 2017 21:17:49 UTC+1, Aymeric Augustin wrote:
>
> On 15 Aug 2017, at 11:44, Gordon Wrigley  > wrote:
>
> I'd like to discuss automatic prefetching in querysets. Specifically 
> automatically doing prefetch_related where needed without the user having 
> to request it.
>
>
>
> Hello,
>
> I'm rather sympathetic to this proposal. Figuring out N + 1 problems in 
> the admin or elsewhere gets old.
>
>
> In addition to everything that was said already, I'd like to point out 
> that Django already has a very similar "magic auto prefetching" behavior in 
> some cases :-)
>
> I'm referring to the admin which calls select_related() on non-nullable 
> foreign keys in the changelist view. The "non-nullable" condition makes 
> that behavior hard to predict — I'd go as far as to call it non 
> deterministic. For details, see slide 54 of 
> https://myks.org/data/20161103-Django_Under_the_Hood-Debugging_Performance.pdf
>  and 
> the audio commentary at https://youtu.be/5fheDDj3oHY?t=2024.
>
>
> The feature proposed here is most useful if it's opt-out because it 
> targets people who aren't aware that the problem even exists — at best they 
> notice that Django is slow and that reminds them vaguely of a rant that 
> explains why ORMs are the worst thing since object oriented programming.
>
> It should kick in only when no select_related or prefetch_related is in 
> effect, to avoid interfering with pre-existing optimizations. It's still 
> easy to construct an example where it would degrade performance but I don't 
> think such situations will be common in practice. Still, there should be a 
> per-queryset opt-out for these cases.
>
> We may want to introduce it with a deprecation path, that is, make it 
> opt-in at first and log a deprecation warning where the behavior would 
> kick-in, so developers who want to disable it can add the per-queryset 
> opt-out.
>
>
> At this point, my main concerns are:
>
> 1) The difficulty of identifying where the queryset originates, given that 
> querysets are lazy. Passing objects around is common; sometimes it can be 
> hard to figure out where an object comes from. It isn't visible in the 
> stack trace. In my opinion this is the strongest argument against the 
> feature.
>
> 2) The lack of this feature for reverse one-to-one relations; it's only 
> implemented for foreign keys. It's hard to tell them apart in Python code. 
> The subtle differences, like return None vs. raise ObjectDoesNotExist when 
> there's no related object, degrade the developer 

Re: Automatic prefetching in querysets

2017-08-17 Thread Gordon Wrigley
I'm not advocating either way on this, but it's probably worth noting that
the context manager proposal is compatible with a queryset level optin/out
and an object level optout.

So you could for example have prefetch_related(auto=None) which can be
explicitly set to True / False and takes it's value from the context
manager when it's None.


On Thu, Aug 17, 2017 at 12:42 AM, Josh Smeaton 
wrote:

> I think there's a lot right with your suggestions here Shai.
>
> It delivers better default behaviour for new projects, does not affect
> existing deployments, and seems pretty easy to enable/disable selectively
> at any level of the stack.
>
> My only concern would be libraries leaning on this behaviour by enabling
> it locally and users being unable to change it. That's only a small concern
> though, and wouldn't prevent me from recommending the proposal.
>
>
> On Thursday, 17 August 2017 09:34:03 UTC+10, Shai Berger wrote:
>>
>> First of all, I think making the auto-prefetch feature available in some
>> form
>> is a great idea. As an opt-in, I would have made use of it on several
>> occasions, and cut days of optimization work to minutes. It's true that
>> this
>> wouldn't achieve the best possible optimization in many cases, but it
>> would be
>> close enough, for a small fraction of the effort.
>>
>> But of the choices that have been suggested here, I find none quite
>> satisfactory. I agree, basically, with almost all of the objections
>> raised on
>> all sides. And I think I have a better suggestion.
>>
>> Rather than a setting (global and essentially constant) or all-local
>> opt-ins,
>> suppose the feature was controlled -- for all querysets -- by a context
>> manager.
>>
>> So, rather than
>>
>> > "qs.prefetch_related(auto=True)", or "with qs.auto_prefetch():",
>>
>> suppose we had
>>
>> with QuerySet.auto_prefetch(active=True):
>>
>> and under that, all querysets would auto-prefetch. Then:
>>
>> - We could put that in a middleware; using that middleware would then,
>> effectively, be a global opt in. We could add the middleware -- perhaps
>> commented-out -- to the new project template, for beginner's sake. Call
>> it
>> PerformanceTrainingWheelsMiddleware, to make people aware that it's
>> something
>> they should grow out of, but make it easily accessible.
>>
>> - We could easily build a decorator for views
>>
>> - We could have local opt-out, at the correct level -- not the objects,
>> but
>> the control flow.
>>
>> For this to work, the flag controlling the feature would need to be
>> thread-
>> local and managed as a stack. But these aren't problems.
>>
>> If (as likely) this suggestion still does not generate a concensus, I
>> would
>> prefer that we follow the path Anssi suggested --
>>
>> > Note that it should be possible to implement this fully as 3rd party
>> > project if we add an easy way to customise how related object fetching
>> is
>> > done. I'm not sure if we can add an easy way for that.
>>
>> Except I don't think "easy" is a requirement here. If we can add a
>> sensible
>> way, that would be enough -- I don't expect many Django users to develop
>> implementations of the feature, only to use them.
>>
>> My 2 cents,
>> Shai.
>>
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Re: Automatic prefetching in querysets

2017-08-16 Thread Josh Smeaton
I think there's a lot right with your suggestions here Shai. 

It delivers better default behaviour for new projects, does not affect 
existing deployments, and seems pretty easy to enable/disable selectively 
at any level of the stack.

My only concern would be libraries leaning on this behaviour by enabling it 
locally and users being unable to change it. That's only a small concern 
though, and wouldn't prevent me from recommending the proposal.

On Thursday, 17 August 2017 09:34:03 UTC+10, Shai Berger wrote:
>
> First of all, I think making the auto-prefetch feature available in some 
> form 
> is a great idea. As an opt-in, I would have made use of it on several 
> occasions, and cut days of optimization work to minutes. It's true that 
> this 
> wouldn't achieve the best possible optimization in many cases, but it 
> would be 
> close enough, for a small fraction of the effort. 
>
> But of the choices that have been suggested here, I find none quite 
> satisfactory. I agree, basically, with almost all of the objections raised 
> on 
> all sides. And I think I have a better suggestion. 
>
> Rather than a setting (global and essentially constant) or all-local 
> opt-ins, 
> suppose the feature was controlled -- for all querysets -- by a context 
> manager. 
>
> So, rather than 
>
> > "qs.prefetch_related(auto=True)", or "with qs.auto_prefetch():", 
>
> suppose we had 
>
> with QuerySet.auto_prefetch(active=True): 
>
> and under that, all querysets would auto-prefetch. Then: 
>
> - We could put that in a middleware; using that middleware would then, 
> effectively, be a global opt in. We could add the middleware -- perhaps 
> commented-out -- to the new project template, for beginner's sake. Call it 
> PerformanceTrainingWheelsMiddleware, to make people aware that it's 
> something 
> they should grow out of, but make it easily accessible. 
>
> - We could easily build a decorator for views 
>
> - We could have local opt-out, at the correct level -- not the objects, 
> but 
> the control flow. 
>
> For this to work, the flag controlling the feature would need to be 
> thread- 
> local and managed as a stack. But these aren't problems. 
>
> If (as likely) this suggestion still does not generate a concensus, I 
> would 
> prefer that we follow the path Anssi suggested -- 
>
> > Note that it should be possible to implement this fully as 3rd party 
> > project if we add an easy way to customise how related object fetching 
> is 
> > done. I'm not sure if we can add an easy way for that. 
>
> Except I don't think "easy" is a requirement here. If we can add a 
> sensible 
> way, that would be enough -- I don't expect many Django users to develop 
> implementations of the feature, only to use them. 
>
> My 2 cents, 
> Shai. 
>

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Re: Automatic prefetching in querysets

2017-08-16 Thread Shai Berger
First of all, I think making the auto-prefetch feature available in some form 
is a great idea. As an opt-in, I would have made use of it on several 
occasions, and cut days of optimization work to minutes. It's true that this 
wouldn't achieve the best possible optimization in many cases, but it would be 
close enough, for a small fraction of the effort.

But of the choices that have been suggested here, I find none quite 
satisfactory. I agree, basically, with almost all of the objections raised on 
all sides. And I think I have a better suggestion.

Rather than a setting (global and essentially constant) or all-local opt-ins, 
suppose the feature was controlled -- for all querysets -- by a context 
manager.

So, rather than

> "qs.prefetch_related(auto=True)", or "with qs.auto_prefetch():", 

suppose we had

with QuerySet.auto_prefetch(active=True):

and under that, all querysets would auto-prefetch. Then:

- We could put that in a middleware; using that middleware would then, 
effectively, be a global opt in. We could add the middleware -- perhaps 
commented-out -- to the new project template, for beginner's sake. Call it 
PerformanceTrainingWheelsMiddleware, to make people aware that it's something 
they should grow out of, but make it easily accessible.

- We could easily build a decorator for views

- We could have local opt-out, at the correct level -- not the objects, but 
the control flow.

For this to work, the flag controlling the feature would need to be thread-
local and managed as a stack. But these aren't problems.

If (as likely) this suggestion still does not generate a concensus, I would 
prefer that we follow the path Anssi suggested -- 

> Note that it should be possible to implement this fully as 3rd party
> project if we add an easy way to customise how related object fetching is
> done. I'm not sure if we can add an easy way for that.

Except I don't think "easy" is a requirement here. If we can add a sensible 
way, that would be enough -- I don't expect many Django users to develop 
implementations of the feature, only to use them.

My 2 cents,
Shai.


Re: Automatic prefetching in querysets

2017-08-16 Thread Gordon Wrigley
Regarding 2, it does work for reverse one-to-one relations.

On Wed, Aug 16, 2017 at 9:17 PM, Aymeric Augustin <
aymeric.augus...@polytechnique.org> wrote:

> On 15 Aug 2017, at 11:44, Gordon Wrigley  wrote:
>
> I'd like to discuss automatic prefetching in querysets. Specifically
> automatically doing prefetch_related where needed without the user having
> to request it.
>
>
>
> Hello,
>
> I'm rather sympathetic to this proposal. Figuring out N + 1 problems in
> the admin or elsewhere gets old.
>
>
> In addition to everything that was said already, I'd like to point out
> that Django already has a very similar "magic auto prefetching" behavior in
> some cases :-)
>
> I'm referring to the admin which calls select_related() on non-nullable
> foreign keys in the changelist view. The "non-nullable" condition makes
> that behavior hard to predict — I'd go as far as to call it non
> deterministic. For details, see slide 54 of https://myks.org/data/
> 20161103-Django_Under_the_Hood-Debugging_Performance.pdf and the audio
> commentary at https://youtu.be/5fheDDj3oHY?t=2024.
>
>
> The feature proposed here is most useful if it's opt-out because it
> targets people who aren't aware that the problem even exists — at best they
> notice that Django is slow and that reminds them vaguely of a rant that
> explains why ORMs are the worst thing since object oriented programming.
>
> It should kick in only when no select_related or prefetch_related is in
> effect, to avoid interfering with pre-existing optimizations. It's still
> easy to construct an example where it would degrade performance but I don't
> think such situations will be common in practice. Still, there should be a
> per-queryset opt-out for these cases.
>
> We may want to introduce it with a deprecation path, that is, make it
> opt-in at first and log a deprecation warning where the behavior would
> kick-in, so developers who want to disable it can add the per-queryset
> opt-out.
>
>
> At this point, my main concerns are:
>
> 1) The difficulty of identifying where the queryset originates, given that
> querysets are lazy. Passing objects around is common; sometimes it can be
> hard to figure out where an object comes from. It isn't visible in the
> stack trace. In my opinion this is the strongest argument against the
> feature.
>
> 2) The lack of this feature for reverse one-to-one relations; it's only
> implemented for foreign keys. It's hard to tell them apart in Python code.
> The subtle differences, like return None vs. raise ObjectDoesNotExist when
> there's no related object, degrade the developer experience.
>
> 3) The strong opinions expressed against the feature. I'm not sure that
> consensus is within reach. If we can't agree that this is an adequate
> amount of magic, we're likely to stick with the status quo. I'd rather not
> have this question decided by a vote of the technical board.
>
>
> In the grand scheme of things, going from "prefetching a related instance
> for an object" to "prefetching related instances for all objects in the
> queryset" isn't that much of a stretch... But I admit it's rather scary to
> make this change for all existing Django projects!
>
>
> Best regards,
>
> --
> Aymeric.
>
>
>
>
> --
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> msgid/django-developers/2D9DCC0F-EAB0-4512-8AEC-
> 08A694DF9074%40polytechnique.org
> 
> .
>
> For more options, visit https://groups.google.com/d/optout.
>

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Re: Automatic prefetching in querysets

2017-08-16 Thread Aymeric Augustin
> On 15 Aug 2017, at 11:44, Gordon Wrigley  wrote:
> 
> I'd like to discuss automatic prefetching in querysets. Specifically 
> automatically doing prefetch_related where needed without the user having to 
> request it.


Hello,

I'm rather sympathetic to this proposal. Figuring out N + 1 problems in the 
admin or elsewhere gets old.


In addition to everything that was said already, I'd like to point out that 
Django already has a very similar "magic auto prefetching" behavior in some 
cases :-)

I'm referring to the admin which calls select_related() on non-nullable foreign 
keys in the changelist view. The "non-nullable" condition makes that behavior 
hard to predict — I'd go as far as to call it non deterministic. For details, 
see slide 54 of 
https://myks.org/data/20161103-Django_Under_the_Hood-Debugging_Performance.pdf 

 and the audio commentary at https://youtu.be/5fheDDj3oHY?t=2024 
.


The feature proposed here is most useful if it's opt-out because it targets 
people who aren't aware that the problem even exists — at best they notice that 
Django is slow and that reminds them vaguely of a rant that explains why ORMs 
are the worst thing since object oriented programming.

It should kick in only when no select_related or prefetch_related is in effect, 
to avoid interfering with pre-existing optimizations. It's still easy to 
construct an example where it would degrade performance but I don't think such 
situations will be common in practice. Still, there should be a per-queryset 
opt-out for these cases.

We may want to introduce it with a deprecation path, that is, make it opt-in at 
first and log a deprecation warning where the behavior would kick-in, so 
developers who want to disable it can add the per-queryset opt-out.


At this point, my main concerns are:

1) The difficulty of identifying where the queryset originates, given that 
querysets are lazy. Passing objects around is common; sometimes it can be hard 
to figure out where an object comes from. It isn't visible in the stack trace. 
In my opinion this is the strongest argument against the feature.

2) The lack of this feature for reverse one-to-one relations; it's only 
implemented for foreign keys. It's hard to tell them apart in Python code. The 
subtle differences, like return None vs. raise ObjectDoesNotExist when there's 
no related object, degrade the developer experience.

3) The strong opinions expressed against the feature. I'm not sure that 
consensus is within reach. If we can't agree that this is an adequate amount of 
magic, we're likely to stick with the status quo. I'd rather not have this 
question decided by a vote of the technical board.


In the grand scheme of things, going from "prefetching a related instance for 
an object" to "prefetching related instances for all objects in the queryset" 
isn't that much of a stretch... But I admit it's rather scary to make this 
change for all existing Django projects!


Best regards,

-- 
Aymeric.




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Re: Automatic prefetching in querysets

2017-08-16 Thread 'Tom Evans' via Django developers (Contributions to Django itself)
Is this opt-{in,out} considered to be a global flag, meant to be
toggled on or off depending on whether it is an "expert" working on
the project or not?

I don't think that would be a good idea, almost all of our projects
have a mix of skill levels, and people move from team to team on a
regular basis. I don't think we have a project that has only been
worked on by senior developers or only worked on by junior developers.

What I think would be exceptionally useful would be to emit loud
warnings when developing (DEBUG=True or runserver) whenever this code
can detect that a prefetch MAY be applicable. A lot of our junior
developers are not as aware about DB structures and SQL performance
(particularly now that they don't design schemas and write DB
queries), so warnings as part of their normal development help trigger
teaching moments. Magically (sometimes) making things faster doesn't
teach them anything.

Turning this feature on/off per app is scary. If we're dealing with
models from AppA, we will get auto pre-fetching, but if we work with
models from AppB, we do not? If we're dealing with those models in the
same module, we will have different behaviour depending on which model
is being used? Please no.

I also think that it might be handy to specify
"qs.prefetch_related(auto=True)", or "with qs.auto_prefetch():", which
would then trigger the newly proposed behaviour. It's like "I want you
to prefetch all the things I use, but I don't know what just yet".
Having said that, I'd also like that to spit out a warning (again, dev
only) that specified what was actually prefetched, because why waste a
DB query in production to determine what we know whilst developing?

Cheers

Tom



On Wed, Aug 16, 2017 at 5:28 PM, Brice Parent  wrote:
> Le 16/08/17 à 14:07, Adam Johnson a écrit :
>>
>> I wouldn't want is to give free optimisations to non-ORM pros
>
>
> Something like 99% of django projects are by non-ORM pros, it can be easy to
> forget that on a mailing list of experts.
>
> I don't count myself as an ORM or SQL expert at all, but I've worked on at
> least 2 projects for which they had contracted such experts to optimise
> their projects once they reached some limits. Those experts only worked for
> them for a limited period of time, and if we changed this behaviour by
> default, I'm 100% sure they wouldn't understanding why their website was
> getting slower with the new release. And they would probably only see the
> performance issue on staging servers if they have some, or in production
> with a full database.
> I admit this can be a great thing for new comers and even small sites. But
> remember that, with or without this functionality, their website will need
> to be optimised in many ways when they start to have more users, and
> removing this 1+N common pitfall (or hiding the problem, as it is not
> completely solved as it's not the more efficient fix) will not be the only
> point they should look at.
> I think the solution for this would simply be to have it as opt-out, not to
> harm any already-working and optimised websites and allow to maintain a
> stable behaviour for 3rd party apps, but make it opt-in to new users just by
> pre-setting some setting in the settings.py file generated by "django-admin
> startproject". It could be a simple boolean setting or a list of apps for
> which we want the auto-fetch feature to be activated on. With this, new
> projects would be optimised unless they want to optimise manually, existing
> projects would still work without decreasing performance or creating memory
> problems in any case. Best of both worlds.
> And for existing non-optimised but existing websites, I prefer the
> status-quo than risking to create problems that might occur on production
> website.
> We could also, as proposed elsewhere in this thread, warn the developers (so
> only when settings.debug is set) when this kind of 1+N queries are executed
> that it could be way more efficient, either by activating the auto-fetch
> functionality or by manually prefetch related data.
>
> - Brice
>
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Re: Automatic prefetching in querysets

2017-08-16 Thread Gordon Wrigley
> It's also likely to require a significant patch

The guts of it are actually surprisingly clean and small. That's probably a
testament to some of the good decisions in the ORM architecture.
Covering one2one in both directions, FK's in the forward direction, ignoring
tests and various optin/out options, it's less than 20 lines, split
reasonably evenly between the three *Descriptor.__get__ functions, and
QuerySet._fetch_all.
Of course lines of code is not a measure of complexity or risk :)


On Wed, Aug 16, 2017 at 2:03 PM, Marc Tamlyn  wrote:

> As an opt in feature, it does sound quite interesting. If over the course
> of a few releases, it proves to be reliable and we don't get tons of
> support requests of it either not working, or causing the opposite problem,
> we can consider moving it to an opt out feature. This makes it easy for
> people to "turn on the magic", but keeps the default safer. It would be
> very easy to write a one line monkeypatch to make it default behaviour.
>
> Django is conservative when it comes to changes to default behaviour, and
> doubly so when the ORM is considered. This will need to have been used
> widely in the wild before it would be considered safe as a default
> behaviour for all projects. This whole process would only take a couple of
> years, which isn't much time in Django release land! I don't mean to
> undermine the belief of the people who think it's definitely an improvement
> in most cases, I just want us to be cautious in how we explore that
> hypothesis.
>
> It's also likely to require a significant patch, and need careful analysis
> before merging. I'd suggest it's a good candidate for a DEP to discuss the
> motivation for the project, and to find a shepherd from the core team
> (Josh? Adam?) to help it land.
>
> On 16 August 2017 at 13:48, Luke Plant  wrote:
>
>> Hi Josh,
>>
>> On 16/08/17 02:26, Josh Smeaton wrote:
>>
>> 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.
>>
>>
>> I wouldn't say it is "optimal behaviour". It is behaviour that is an
>> optimization for some use cases - common ones, I'd agree - but not all. In
>> fact it is not even the best optimization - it performs less well in many
>> cases than a manual `select_related`.
>>
>> We don't know how many sites would be affected by the opposite default,
>> because we aren't putting people in that situation. Generating potentially
>> large queries (i.e. ones that return lots of results) is going to make
>> someone's life a lot harder, and can even crash processes (out of memory
>> errors), or cause massive slowdown due to insufficient memory and swapping.
>> I have had these kind of issues in production systems, and sometimes the
>> answer is to prefetch *less* - which is why things like `iterator()` exist,
>> because sometimes you *don't* want to load it all into memory upfront.
>>
>> 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.
>>
>>
>> This is far from the only place where we expect users to be conscious of
>> what has to go on at the database level. For example, we expect users to
>> use `.count()` and `.exists()` where appropriate, and not use them where
>> not appropriate - see https://docs.djangoproject.com
>> /en/1.11/topics/db/optimization/#don-t-overuse-count-and-exists
>>
>> This is an example of doing it right. We could have 'intelligently' made
>> `__len__()` do `count()`, but this introduces queries that the user did not
>> ask for, and that's hard for developers to predict. That whole section of
>> the docs on DB optimisation depends on the possibility of understanding
>> things at a DB level, and understanding how QuerySets behave, and that they
>> only do the queries that you ask them to do. The more magic we build in,
>> the harder we make life for people trying to optimize database access.
>>
>> If we have an `auto_prefetch` method that has to be called explicitly,
>> then we are allowing users who know less about databases to get something
>> that works OK for many situations. But having it on by default makes
>> optimization harder and adds unwelcome surprises.
>>
>>
>> I'd be more in favour of throwing an error on
>> 

Re: Automatic prefetching in querysets

2017-08-16 Thread Tom Forbes
Some quicker changes that have been brought up here could be implemented in
the interim, like adding intelligent warnings.

Someone brought up a great point about how the ORM hides most SQL
terminology from the user, but still requires the knowledge of the
difference between prefetch and select related.

Perhaps a good idea would be to create a generic 'related' method on the
queryset that handles both cases? Hiding the differences between how m2m
and foreign keys are fetched could make them seem less complicated and more
approachable to new users?

On 16 Aug 2017 14:04, "Marc Tamlyn"  wrote:

As an opt in feature, it does sound quite interesting. If over the course
of a few releases, it proves to be reliable and we don't get tons of
support requests of it either not working, or causing the opposite problem,
we can consider moving it to an opt out feature. This makes it easy for
people to "turn on the magic", but keeps the default safer. It would be
very easy to write a one line monkeypatch to make it default behaviour.

Django is conservative when it comes to changes to default behaviour, and
doubly so when the ORM is considered. This will need to have been used
widely in the wild before it would be considered safe as a default
behaviour for all projects. This whole process would only take a couple of
years, which isn't much time in Django release land! I don't mean to
undermine the belief of the people who think it's definitely an improvement
in most cases, I just want us to be cautious in how we explore that
hypothesis.

It's also likely to require a significant patch, and need careful analysis
before merging. I'd suggest it's a good candidate for a DEP to discuss the
motivation for the project, and to find a shepherd from the core team
(Josh? Adam?) to help it land.

On 16 August 2017 at 13:48, Luke Plant  wrote:

> Hi Josh,
>
> On 16/08/17 02:26, Josh Smeaton wrote:
>
> 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.
>
>
> I wouldn't say it is "optimal behaviour". It is behaviour that is an
> optimization for some use cases - common ones, I'd agree - but not all. In
> fact it is not even the best optimization - it performs less well in many
> cases than a manual `select_related`.
>
> We don't know how many sites would be affected by the opposite default,
> because we aren't putting people in that situation. Generating potentially
> large queries (i.e. ones that return lots of results) is going to make
> someone's life a lot harder, and can even crash processes (out of memory
> errors), or cause massive slowdown due to insufficient memory and swapping.
> I have had these kind of issues in production systems, and sometimes the
> answer is to prefetch *less* - which is why things like `iterator()` exist,
> because sometimes you *don't* want to load it all into memory upfront.
>
> 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.
>
>
> This is far from the only place where we expect users to be conscious of
> what has to go on at the database level. For example, we expect users to
> use `.count()` and `.exists()` where appropriate, and not use them where
> not appropriate - see https://docs.djangoproject.com
> /en/1.11/topics/db/optimization/#don-t-overuse-count-and-exists
>
> This is an example of doing it right. We could have 'intelligently' made
> `__len__()` do `count()`, but this introduces queries that the user did not
> ask for, and that's hard for developers to predict. That whole section of
> the docs on DB optimisation depends on the possibility of understanding
> things at a DB level, and understanding how QuerySets behave, and that they
> only do the queries that you ask them to do. The more magic we build in,
> the harder we make life for people trying to optimize database access.
>
> If we have an `auto_prefetch` method that has to be called explicitly,
> then we are allowing users who know less about databases to get something
> that works OK for many situations. But having it on by default makes
> optimization harder and adds unwelcome surprises.
>
>
> 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 

Re: Automatic prefetching in querysets

2017-08-16 Thread Marc Tamlyn
As an opt in feature, it does sound quite interesting. If over the course
of a few releases, it proves to be reliable and we don't get tons of
support requests of it either not working, or causing the opposite problem,
we can consider moving it to an opt out feature. This makes it easy for
people to "turn on the magic", but keeps the default safer. It would be
very easy to write a one line monkeypatch to make it default behaviour.

Django is conservative when it comes to changes to default behaviour, and
doubly so when the ORM is considered. This will need to have been used
widely in the wild before it would be considered safe as a default
behaviour for all projects. This whole process would only take a couple of
years, which isn't much time in Django release land! I don't mean to
undermine the belief of the people who think it's definitely an improvement
in most cases, I just want us to be cautious in how we explore that
hypothesis.

It's also likely to require a significant patch, and need careful analysis
before merging. I'd suggest it's a good candidate for a DEP to discuss the
motivation for the project, and to find a shepherd from the core team
(Josh? Adam?) to help it land.

On 16 August 2017 at 13:48, Luke Plant  wrote:

> Hi Josh,
>
> On 16/08/17 02:26, Josh Smeaton wrote:
>
> 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.
>
>
> I wouldn't say it is "optimal behaviour". It is behaviour that is an
> optimization for some use cases - common ones, I'd agree - but not all. In
> fact it is not even the best optimization - it performs less well in many
> cases than a manual `select_related`.
>
> We don't know how many sites would be affected by the opposite default,
> because we aren't putting people in that situation. Generating potentially
> large queries (i.e. ones that return lots of results) is going to make
> someone's life a lot harder, and can even crash processes (out of memory
> errors), or cause massive slowdown due to insufficient memory and swapping.
> I have had these kind of issues in production systems, and sometimes the
> answer is to prefetch *less* - which is why things like `iterator()` exist,
> because sometimes you *don't* want to load it all into memory upfront.
>
> 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.
>
>
> This is far from the only place where we expect users to be conscious of
> what has to go on at the database level. For example, we expect users to
> use `.count()` and `.exists()` where appropriate, and not use them where
> not appropriate - see https://docs.djangoproject.com/en/1.11/topics/db/
> optimization/#don-t-overuse-count-and-exists
>
> This is an example of doing it right. We could have 'intelligently' made
> `__len__()` do `count()`, but this introduces queries that the user did not
> ask for, and that's hard for developers to predict. That whole section of
> the docs on DB optimisation depends on the possibility of understanding
> things at a DB level, and understanding how QuerySets behave, and that they
> only do the queries that you ask them to do. The more magic we build in,
> the harder we make life for people trying to optimize database access.
>
> If we have an `auto_prefetch` method that has to be called explicitly,
> then we are allowing users who know less about databases to get something
> that works OK for many situations. But having it on by default makes
> optimization harder and adds unwelcome surprises.
>
>
> 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.
>
>
> I have some sympathy with this, but I doubt it is somewhere we can go from
> the current ORM and Django ecosystem as a default - but see below.
>
>
> 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 

Re: Automatic prefetching in querysets

2017-08-16 Thread Luke Plant

Hi Josh,


On 16/08/17 02:26, Josh Smeaton wrote:
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.


I wouldn't say it is "optimal behaviour". It is behaviour that is an 
optimization for some use cases - common ones, I'd agree - but not all. 
In fact it is not even the best optimization - it performs less well in 
many cases than a manual `select_related`.


We don't know how many sites would be affected by the opposite default, 
because we aren't putting people in that situation. Generating 
potentially large queries (i.e. ones that return lots of results) is 
going to make someone's life a lot harder, and can even crash processes 
(out of memory errors), or cause massive slowdown due to insufficient 
memory and swapping. I have had these kind of issues in production 
systems, and sometimes the answer is to prefetch *less* - which is why 
things like `iterator()` exist, because sometimes you *don't* want to 
load it all into memory upfront.


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.


This is far from the only place where we expect users to be conscious of 
what has to go on at the database level. For example, we expect users to 
use `.count()` and `.exists()` where appropriate, and not use them where 
not appropriate - see 
https://docs.djangoproject.com/en/1.11/topics/db/optimization/#don-t-overuse-count-and-exists


This is an example of doing it right. We could have 'intelligently' made 
`__len__()` do `count()`, but this introduces queries that the user did 
not ask for, and that's hard for developers to predict. That whole 
section of the docs on DB optimisation depends on the possibility of 
understanding things at a DB level, and understanding how QuerySets 
behave, and that they only do the queries that you ask them to do. The 
more magic we build in, the harder we make life for people trying to 
optimize database access.


If we have an `auto_prefetch` method that has to be called explicitly, 
then we are allowing users who know less about databases to get 
something that works OK for many situations. But having it on by default 
makes optimization harder and adds unwelcome surprises.




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.


I have some sympathy with this, but I doubt it is somewhere we can go 
from the current ORM and Django ecosystem as a default - but see below.




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 completely agree that visibility of this problem is a major issue, and 
would really welcome work on improving this, especially in DEBUG mode. 
One option might be a method that replaces lazy loads with exceptions. 
This would force you to add the required `prefetch_related` or 
`select_related` calls. You could have:


.lazy_load(None)   # exception on accessing any non-loaded FK objects

.lazy_load('my_fk1', 'my_fk2')  # exceptions on accessing any unloaded 
FK objects apart from the named ones


.lazy_load('__any__')   # cancel the above, restore default behaviour

This (or something like it) would be a different way to tackle the 
problem - just throwing it out. You could have a Meta option to do 
`.lazy_load(None)` by default for a Model.  I've no idea how practical 
it would be to wire this all up so that is works correctly, and with 
nested objects etc.


This would be a bit of a departure from the current ORM, especially if 
we wanted to migrate to it being the default behaviour. If we are 
thinking long term, we might have to move to something like this if the 
future is async. Lazy-loading ORMs are not very friendly to async code, 
but an ORM where you must specify a set of queries to execute up-front 
might be more of a possibility.


I think a better question to ask is:

- How many 

Re: Automatic prefetching in querysets

2017-08-16 Thread Anssi Kääriäinen
On Wednesday, August 16, 2017 at 2:27:11 PM UTC+3, Josh Smeaton wrote:
>
> It won't affect experienced users. They'll read the release notes, see 
>> that this change has been implemented, and either go and delete a bunch of 
>> prefetch_related() calls, grumble a bit and turn auto-prefetch off globally 
>> or just file it away as another fact they know about the Django ORM.
>>
>
> This is, for me, the biggest point in favour of having it opt out. It'll 
> barely affect anyone who is really experienced, but can provide such a 
> large upside for others.
>

There is huge potential for this feature going very bad for performance. 
For example, this common anti-pattern would immediately cause interesting 
issues:

if qs:
qs[0].do_something()

then if do_something() does access a couple of relations, you'll load data 
for the whole queryset where you only need it for the first one. So, at 
least make this opt-in instead of opt-out.

Doing this for "select_related" cases on opt-in basis seems like a 
plausible idea. Doing this for reverse foreign key or m2m cases where you 
need to use prefetch_related seems complex to implement, and just for that 
reason I'd avoid the prefetch related case at least initially.

Note that it should be possible to implement this fully as 3rd party 
project if we add an easy way to customise how related object fetching is 
done. I'm not sure if we can add an easy way for that.

By the way, a very efficient test for N+1 queries issue is to do something 
like this:

def test_nplusone_queries(self):
self.create_object()
with self.count_queries() as cnt:
self.client.get('object_listing/')
self.create_object()
with self.assertNumQueries(cnt):
self.client.get('object_listing/')

that is, you test the same list view with one and two objects. If there was 
an easy way to write tests like this, that would make n+1 queries problems 
a lot easier to detect. Maybe something to make this kind of test trivial 
to write could be in Django? Something like 
self.assertListViewScales(object_creator, 'view_name')?

 - Anssi

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Re: Automatic prefetching in querysets

2017-08-16 Thread Adam Johnson
>
> I wouldn't want is to give free optimisations to non-ORM pros
>

Something like 99% of django projects are by non-ORM pros, it can be easy
to forget that on a mailing list of experts.

On 16 August 2017 at 12:27, Josh Smeaton  wrote:

> It won't affect experienced users. They'll read the release notes, see
>> that this change has been implemented, and either go and delete a bunch of
>> prefetch_related() calls, grumble a bit and turn auto-prefetch off globally
>> or just file it away as another fact they know about the Django ORM.
>>
>
> This is, for me, the biggest point in favour of having it opt out. It'll
> barely affect anyone who is really experienced, but can provide such a
> large upside for others.
>
> --
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> msgid/django-developers/3bf2f56e-8534-404b-8a70-
> 12a657be0514%40googlegroups.com
> 
> .
>
> For more options, visit https://groups.google.com/d/optout.
>



-- 
Adam

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Re: Automatic prefetching in querysets

2017-08-16 Thread Josh Smeaton

>
> It won't affect experienced users. They'll read the release notes, see 
> that this change has been implemented, and either go and delete a bunch of 
> prefetch_related() calls, grumble a bit and turn auto-prefetch off globally 
> or just file it away as another fact they know about the Django ORM.
>

This is, for me, the biggest point in favour of having it opt out. It'll 
barely affect anyone who is really experienced, but can provide such a 
large upside for others.

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Re: Automatic prefetching in querysets

2017-08-15 Thread Alexander Hill
I think this is an excellent suggestion.

It seems generally accepted in this thread that although there are cases
where this would hurt performance, it would on average solve more problems
than it creates. The debate seems to be more whether or not it's "right"
for the ORM to behave in this magical way. With that in mind...

It won't affect experienced users. They'll read the release notes, see that
this change has been implemented, and either go and delete a bunch of
prefetch_related() calls, grumble a bit and turn auto-prefetch off globally
or just file it away as another fact they know about the Django ORM.

For beginners, this can defer the pain of learning about N+1 problems,
potentially forever depending on the scale of the project. Ultimately
Django's job isn't to teach hard lessons about SQL gotchas, it's to make it
easy to make a nice website. This proposal would reduce both average load
time of Django pages and debugging time, at the cost of the average Django
developer being a little more ignorant about what queries the ORM
generates. I think that's a good trade and in line with the goals of the
project.

Django's ORM isn't SQLAlchemy - it's built on high-level concepts, designed
to be relatively beginner-friendly, and already pretty magical.
select_related() and prefetch_related() are somewhat awkward plugs for a
leaky abstraction, and IMO this is just a better plug.

Alex


On Wed, 16 Aug 2017 at 12:12 Cristiano Coelho 
wrote:

> I would rather have warnings as well, adding more magical behavior is bad
> and might even degrade performance on some cases, automatically selecting a
> bunch of data that "might" be used is bad, and specially considering how
> slow python is, accidentally loading/building 1k+ objects when maybe only
> one of them is used would be as bad as doing 1k+ queries.
>
> If the systems you are building are that large and complicated you can't
> have people with 0 SQL knowledge doing stuff neither! So many things to
> tweak, indexes, data denormalization, proper joins here and there, unique
> constraints, locks and race conditions, someone attempting to code
> something that's not a blog or hello world really needs to know a bit about
> all of that.
>
>
>
> El martes, 15 de agosto de 2017, 6:44:19 (UTC-3), Gordon Wrigley escribió:
>>
>> 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
>>  
>> 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 
>> (which I was involved in creating) and 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Cristiano Coelho
I would rather have warnings as well, adding more magical behavior is bad 
and might even degrade performance on some cases, automatically selecting a 
bunch of data that "might" be used is bad, and specially considering how 
slow python is, accidentally loading/building 1k+ objects when maybe only 
one of them is used would be as bad as doing 1k+ queries.

If the systems you are building are that large and complicated you can't 
have people with 0 SQL knowledge doing stuff neither! So many things to 
tweak, indexes, data denormalization, proper joins here and there, unique 
constraints, locks and race conditions, someone attempting to code 
something that's not a blog or hello world really needs to know a bit about 
all of that.


El martes, 15 de agosto de 2017, 6:44:19 (UTC-3), Gordon Wrigley escribió:
>
> 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 
>  
> 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  
> (which I was involved in creating) and 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Curtis Maloney

The 2 goals of a famework:
- protect you from the tedious things
- protect you from the dangerous things

N+1 queries would be in the 'dangerous' category, IMHO, and 'detecting 
causes of N+1 queries' is in the 'tedious'.


If we can at least add some DEBUG flagged machinery to detect and warn 
of potential prefetch/select candidates, it would be a big win.


--
C

On 16/08/17 09:26, Josh Smeaton wrote:
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"  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, 

Re: Automatic prefetching in querysets

2017-08-15 Thread Josh Smeaton
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"  
> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Anthony King
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"  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 
> 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
>>  
>> 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 
>> (which I was involved in creating) and 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Adam Johnson
>
> 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.


I think Django automatically fetching data from the database on access is
already magical, it certainly surprises new users (in good and bad ways)
when they first come across it, especially if they've only worked with SQL
before and not any other ORM's.

This is not doing "extra" queries, just fetching a bit more data. In the
most likely case, there are far fewer queries - N+1 will instead become 2.
I think it's a reasonable assumption that if choices_queryset[0].question
is accessed, choices_queryset[1].question up to
choices_queryset[n].question will
be accessed too. The current behaviour is effectively assuming that it's
completely *unlikely*.

On 15 August 2017 at 23:24, Adam Johnson  wrote:

> Adam/Gordon, I'm interested in hearing how these changes led you to
>> discovering stale prefetches?
>
>
> We removed all the manual prefetches in admin get_queryset() methods,
> added the auto_prefetch_related call, and regenerated the performance
> records from django-perf-rec tests that hit the admin classes on the list
> view, detail view, etc. Some queries disappeared and on inspection it was
> obvious they were for features since removed e.g. a related field no longer
> shown on the list view.
>
> On 15 August 2017 at 23:05, Josh Smeaton  wrote:
>
>> I'm in favour of *some* automated way of handling these prefetches,
>> whether it's opt-in or opt-out there should be some mechanism for
>> protection. Preferably with loud logging that directs users to the source
>> of the automated hand-holding so they have an opportunity to disable or
>> fine tune the queries. Not all Django devs are ORM/Database/Python experts
>> - some are frontend devs just trying to get by. I know that this kind of
>> proposed behaviour would have both saved our site from massive performance
>> issues, but also likely guided these same devs to the source of potential
>> issues.
>>
>> Agreed that prefetch_related is strictly better than not, and is
>> acceptable in the absence of select_related.
>>
>> I think keeping the code back and focussing on the idea first is a good
>> one. I think I'd like to know whether people thought that opt-in or opt-out
>> behaviour would be best?
>>
>> For me - there are a few cases where automatically prefetching would
>> *maybe* be the wrong thing to do. In the vast majority of cases it'd be
>> better than the default of having nothing.
>>
>> A few concerns:
>>
>> - Be careful of `.iterator()` queries (that can't use prefetch)
>> - Could we warn of reverse M2M/ForeignKey at least?
>>
>> Adam/Gordon, I'm interested in hearing how these changes led you to
>> discovering stale prefetches?
>>
>> On Wednesday, 16 August 2017 07:30:10 UTC+10, Adam Johnson wrote:
>>>
>>> I'm biased here, Gordon was my boss for nearly three years, 2014-2016.
>>>
>>> I'm in favour of adding the auto-prefetching, I've seen it work. It was
>>> created around midway through last year and applied to our application's
>>> Admin interface, which was covered in tests with *django-perf-rec*.
>>> After adding the automatic prefetch, we not only identified some existing
>>> N+1 query problems that hadn't been picked up (they were there in the
>>> performance record files, just there were so many queries humans had missed
>>> reading some), we also found a number of stale prefetch queries that could
>>> be removed because the data they fetched wasn't being used. Additionally
>>> not all of the admin interface was perf-rec-tested/optimized so some pages
>>> were "just faster" with no extra effort.
>>>
>>> I think there's a case for adding it to core - releasing as a third
>>> party package would make it difficult to use since it requires changes -
>>> mostly small - to QuerySet, ForeignKey, and the descriptor for ForeignKey.
>>> Users of such a package would have to use subclasses of all of these, the
>>> trickiest being ForeignKey since it triggers migrations... We had the
>>> luxury in our codebase of already having these subclasses for other
>>> customizations, so it was easier to roll out.
>>>
>>> On 15 August 2017 at 20:35, Gordon Wrigley  wrote:
>>>
 The warnings you propose would certainly be an improvement on the
 status quo.
 However for that to be a complete solution Django would also need to
 detect places where there are redundant prefetch_relateds.

 Additionally tools like the Admin and DRF would need to provide
 adequate hooks for inserting these calls.
 For example ModelAdmin.get_queryset is not really granular enough as
 it's used by both the list and detail views which might touch quite
 different sets of fields. (Although in practice what you generally do is
 optimize the list 

Re: Automatic prefetching in querysets

2017-08-15 Thread Adam Johnson
>
> Adam/Gordon, I'm interested in hearing how these changes led you to
> discovering stale prefetches?


We removed all the manual prefetches in admin get_queryset() methods, added
the auto_prefetch_related call, and regenerated the performance records
from django-perf-rec tests that hit the admin classes on the list view,
detail view, etc. Some queries disappeared and on inspection it was obvious
they were for features since removed e.g. a related field no longer shown
on the list view.

On 15 August 2017 at 23:05, Josh Smeaton  wrote:

> I'm in favour of *some* automated way of handling these prefetches,
> whether it's opt-in or opt-out there should be some mechanism for
> protection. Preferably with loud logging that directs users to the source
> of the automated hand-holding so they have an opportunity to disable or
> fine tune the queries. Not all Django devs are ORM/Database/Python experts
> - some are frontend devs just trying to get by. I know that this kind of
> proposed behaviour would have both saved our site from massive performance
> issues, but also likely guided these same devs to the source of potential
> issues.
>
> Agreed that prefetch_related is strictly better than not, and is
> acceptable in the absence of select_related.
>
> I think keeping the code back and focussing on the idea first is a good
> one. I think I'd like to know whether people thought that opt-in or opt-out
> behaviour would be best?
>
> For me - there are a few cases where automatically prefetching would
> *maybe* be the wrong thing to do. In the vast majority of cases it'd be
> better than the default of having nothing.
>
> A few concerns:
>
> - Be careful of `.iterator()` queries (that can't use prefetch)
> - Could we warn of reverse M2M/ForeignKey at least?
>
> Adam/Gordon, I'm interested in hearing how these changes led you to
> discovering stale prefetches?
>
> On Wednesday, 16 August 2017 07:30:10 UTC+10, Adam Johnson wrote:
>>
>> I'm biased here, Gordon was my boss for nearly three years, 2014-2016.
>>
>> I'm in favour of adding the auto-prefetching, I've seen it work. It was
>> created around midway through last year and applied to our application's
>> Admin interface, which was covered in tests with *django-perf-rec*.
>> After adding the automatic prefetch, we not only identified some existing
>> N+1 query problems that hadn't been picked up (they were there in the
>> performance record files, just there were so many queries humans had missed
>> reading some), we also found a number of stale prefetch queries that could
>> be removed because the data they fetched wasn't being used. Additionally
>> not all of the admin interface was perf-rec-tested/optimized so some pages
>> were "just faster" with no extra effort.
>>
>> I think there's a case for adding it to core - releasing as a third party
>> package would make it difficult to use since it requires changes - mostly
>> small - to QuerySet, ForeignKey, and the descriptor for ForeignKey. Users
>> of such a package would have to use subclasses of all of these, the
>> trickiest being ForeignKey since it triggers migrations... We had the
>> luxury in our codebase of already having these subclasses for other
>> customizations, so it was easier to roll out.
>>
>> On 15 August 2017 at 20:35, Gordon Wrigley  wrote:
>>
>>> The warnings you propose would certainly be an improvement on the status
>>> quo.
>>> However for that to be a complete solution Django would also need to
>>> detect places where there are redundant prefetch_relateds.
>>>
>>> Additionally tools like the Admin and DRF would need to provide adequate
>>> hooks for inserting these calls.
>>> For example ModelAdmin.get_queryset is not really granular enough as
>>> it's used by both the list and detail views which might touch quite
>>> different sets of fields. (Although in practice what you generally do is
>>> optimize the list view as that's the one that tends to explode)
>>>
>>> That aside I sincerely believe that the proposed approach is superior to
>>> the current default behavior in the majority of cases and further more
>>> doesn't fail as badly as the current behavior when it's not appropriate. I
>>> expect that if implemented as an option then in time that belief would
>>> prove itself.
>>>
>>> On Tue, Aug 15, 2017 at 8:17 PM, Tom Forbes  wrote:
>>>
 Exploding query counts are definitely a pain point in Django, anything
 to improve that is definitely worth considering. They have been a problem
 in all Django projects I have seen.

 However I think the correct solution is for developers to correctly add
 select/prefetch calls. There is no general solution for automatically
 applying them that works for enough cases, and i think adding such a method
 to querysets would be used incorrectly and too often.

 Perhaps a better solution would be for Django to detect these O(n)
 query cases and 

Re: Automatic prefetching in querysets

2017-08-15 Thread Sean Brant
I wonder if a solution similar to [1] from the rails world would satisfy
this request. Rather then doing anything 'magical' we instead log when we
detect things like accessing a related model that has not been pre-fetched.

[1] https://github.com/flyerhzm/bullet

On Tue, Aug 15, 2017 at 5:14 PM, Luke Plant  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 
> 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
>>  
>> 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 
>> (which I was involved in creating) and 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Luke Plant
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 > 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
 
when
there are 100 questions each with 5 choices for a total of 500
choices.

Default
|
forchoice inChoice.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
|
forchoice inChoice.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
|
forchoice inChoice.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
 (which I was involved
in creating) and 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Josh Smeaton
I'm in favour of *some* automated way of handling these prefetches, whether 
it's opt-in or opt-out there should be some mechanism for protection. 
Preferably with loud logging that directs users to the source of the 
automated hand-holding so they have an opportunity to disable or fine tune 
the queries. Not all Django devs are ORM/Database/Python experts - some are 
frontend devs just trying to get by. I know that this kind of proposed 
behaviour would have both saved our site from massive performance issues, 
but also likely guided these same devs to the source of potential issues.

Agreed that prefetch_related is strictly better than not, and is acceptable 
in the absence of select_related.

I think keeping the code back and focussing on the idea first is a good 
one. I think I'd like to know whether people thought that opt-in or opt-out 
behaviour would be best?

For me - there are a few cases where automatically prefetching would 
*maybe* be the wrong thing to do. In the vast majority of cases it'd be 
better than the default of having nothing.

A few concerns:

- Be careful of `.iterator()` queries (that can't use prefetch)
- Could we warn of reverse M2M/ForeignKey at least?

Adam/Gordon, I'm interested in hearing how these changes led you to 
discovering stale prefetches?

On Wednesday, 16 August 2017 07:30:10 UTC+10, Adam Johnson wrote:
>
> I'm biased here, Gordon was my boss for nearly three years, 2014-2016.
>
> I'm in favour of adding the auto-prefetching, I've seen it work. It was 
> created around midway through last year and applied to our application's 
> Admin interface, which was covered in tests with *django-perf-rec*. After 
> adding the automatic prefetch, we not only identified some existing N+1 
> query problems that hadn't been picked up (they were there in the 
> performance record files, just there were so many queries humans had missed 
> reading some), we also found a number of stale prefetch queries that could 
> be removed because the data they fetched wasn't being used. Additionally 
> not all of the admin interface was perf-rec-tested/optimized so some pages 
> were "just faster" with no extra effort.
>
> I think there's a case for adding it to core - releasing as a third party 
> package would make it difficult to use since it requires changes - mostly 
> small - to QuerySet, ForeignKey, and the descriptor for ForeignKey. Users 
> of such a package would have to use subclasses of all of these, the 
> trickiest being ForeignKey since it triggers migrations... We had the 
> luxury in our codebase of already having these subclasses for other 
> customizations, so it was easier to roll out.
>
> On 15 August 2017 at 20:35, Gordon Wrigley  > wrote:
>
>> The warnings you propose would certainly be an improvement on the status 
>> quo.
>> However for that to be a complete solution Django would also need to 
>> detect places where there are redundant prefetch_relateds.
>>
>> Additionally tools like the Admin and DRF would need to provide adequate 
>> hooks for inserting these calls.
>> For example ModelAdmin.get_queryset is not really granular enough as it's 
>> used by both the list and detail views which might touch quite different 
>> sets of fields. (Although in practice what you generally do is optimize the 
>> list view as that's the one that tends to explode)
>>
>> That aside I sincerely believe that the proposed approach is superior to 
>> the current default behavior in the majority of cases and further more 
>> doesn't fail as badly as the current behavior when it's not appropriate. I 
>> expect that if implemented as an option then in time that belief would 
>> prove itself.
>>
>> On Tue, Aug 15, 2017 at 8:17 PM, Tom Forbes > > wrote:
>>
>>> Exploding query counts are definitely a pain point in Django, anything 
>>> to improve that is definitely worth considering. They have been a problem 
>>> in all Django projects I have seen.
>>>
>>> However I think the correct solution is for developers to correctly add 
>>> select/prefetch calls. There is no general solution for automatically 
>>> applying them that works for enough cases, and i think adding such a method 
>>> to querysets would be used incorrectly and too often. 
>>>
>>> Perhaps a better solution would be for Django to detect these O(n) query 
>>> cases and display intelligent warnings, with suggestions as to the correct 
>>> select/prefetch calls to add. When debug mode is enabled we could detect 
>>> repeated foreign key referencing from the same source.
>>>
>>> On 15 Aug 2017 19:44, "Gordon Wrigley" >> > wrote:
>>>
>>> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Adam Johnson
I'm biased here, Gordon was my boss for nearly three years, 2014-2016.

I'm in favour of adding the auto-prefetching, I've seen it work. It was
created around midway through last year and applied to our application's
Admin interface, which was covered in tests with *django-perf-rec*. After
adding the automatic prefetch, we not only identified some existing N+1
query problems that hadn't been picked up (they were there in the
performance record files, just there were so many queries humans had missed
reading some), we also found a number of stale prefetch queries that could
be removed because the data they fetched wasn't being used. Additionally
not all of the admin interface was perf-rec-tested/optimized so some pages
were "just faster" with no extra effort.

I think there's a case for adding it to core - releasing as a third party
package would make it difficult to use since it requires changes - mostly
small - to QuerySet, ForeignKey, and the descriptor for ForeignKey. Users
of such a package would have to use subclasses of all of these, the
trickiest being ForeignKey since it triggers migrations... We had the
luxury in our codebase of already having these subclasses for other
customizations, so it was easier to roll out.

On 15 August 2017 at 20:35, Gordon Wrigley  wrote:

> The warnings you propose would certainly be an improvement on the status
> quo.
> However for that to be a complete solution Django would also need to
> detect places where there are redundant prefetch_relateds.
>
> Additionally tools like the Admin and DRF would need to provide adequate
> hooks for inserting these calls.
> For example ModelAdmin.get_queryset is not really granular enough as it's
> used by both the list and detail views which might touch quite different
> sets of fields. (Although in practice what you generally do is optimize the
> list view as that's the one that tends to explode)
>
> That aside I sincerely believe that the proposed approach is superior to
> the current default behavior in the majority of cases and further more
> doesn't fail as badly as the current behavior when it's not appropriate. I
> expect that if implemented as an option then in time that belief would
> prove itself.
>
> On Tue, Aug 15, 2017 at 8:17 PM, Tom Forbes  wrote:
>
>> Exploding query counts are definitely a pain point in Django, anything to
>> improve that is definitely worth considering. They have been a problem in
>> all Django projects I have seen.
>>
>> However I think the correct solution is for developers to correctly add
>> select/prefetch calls. There is no general solution for automatically
>> applying them that works for enough cases, and i think adding such a method
>> to querysets would be used incorrectly and too often.
>>
>> Perhaps a better solution would be for Django to detect these O(n) query
>> cases and display intelligent warnings, with suggestions as to the correct
>> select/prefetch calls to add. When debug mode is enabled we could detect
>> repeated foreign key referencing from the same source.
>>
>> On 15 Aug 2017 19:44, "Gordon Wrigley"  wrote:
>>
>> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Gordon Wrigley
The warnings you propose would certainly be an improvement on the status
quo.
However for that to be a complete solution Django would also need to detect
places where there are redundant prefetch_relateds.

Additionally tools like the Admin and DRF would need to provide adequate
hooks for inserting these calls.
For example ModelAdmin.get_queryset is not really granular enough as it's
used by both the list and detail views which might touch quite different
sets of fields. (Although in practice what you generally do is optimize the
list view as that's the one that tends to explode)

That aside I sincerely believe that the proposed approach is superior to
the current default behavior in the majority of cases and further more
doesn't fail as badly as the current behavior when it's not appropriate. I
expect that if implemented as an option then in time that belief would
prove itself.

On Tue, Aug 15, 2017 at 8:17 PM, Tom Forbes  wrote:

> Exploding query counts are definitely a pain point in Django, anything to
> improve that is definitely worth considering. They have been a problem in
> all Django projects I have seen.
>
> However I think the correct solution is for developers to correctly add
> select/prefetch calls. There is no general solution for automatically
> applying them that works for enough cases, and i think adding such a method
> to querysets would be used incorrectly and too often.
>
> Perhaps a better solution would be for Django to detect these O(n) query
> cases and display intelligent warnings, with suggestions as to the correct
> select/prefetch calls to add. When debug mode is enabled we could detect
> repeated foreign key referencing from the same source.
>
> On 15 Aug 2017 19:44, "Gordon Wrigley"  wrote:
>
> 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 
> 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 
>> 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

Re: Automatic prefetching in querysets

2017-08-15 Thread Gordon Wrigley
I didn't answer your questions directly. Sorry for the quoting but it's the
easiest way to deal with a half dozen questions.

> How would possible prefetches be identified?

Wherever we currently automatically fetch a foreign key value.

> What happens when an initial loop in a view requires one prefetch, but a
subsequent loop in a template requires some other prefetch?

They each do whatever prefetch they need, just as a human optimizing this
would add two prefetch clauses.

> What about nested loops resulting in nested prefetches?

Nested loops only really come up when you are dealing with RelatedManagers
which are outside the scope of this. Or did you have some other nested loop
case in mind?

>  I would argue that correctly setting up and maintaining appropriate
prefetches and selects is a necessary part of working with an ORM.

Having been lead engineer on a code base of ~100,000 lines with over 100
calls to prefetch_related and a lot of tests specifically for finding
missing ones I'd argue it's one of the worst aspects of working with
Djangos ORM at non trivial scale.

> Do you know of any other ORMs which attempt similar magical optimisations?


I don't, but unlike Django where I have years of experience I have next to
no experience with other ORM's.

Regards G

On Tue, Aug 15, 2017 at 7:44 PM, Gordon Wrigley 
wrote:

> 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 
> 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 
>> 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
>>>  
>>> 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
>>> 

Re: Automatic prefetching in querysets

2017-08-15 Thread Tom Forbes
Exploding query counts are definitely a pain point in Django, anything to
improve that is definitely worth considering. They have been a problem in
all Django projects I have seen.

However I think the correct solution is for developers to correctly add
select/prefetch calls. There is no general solution for automatically
applying them that works for enough cases, and i think adding such a method
to querysets would be used incorrectly and too often.

Perhaps a better solution would be for Django to detect these O(n) query
cases and display intelligent warnings, with suggestions as to the correct
select/prefetch calls to add. When debug mode is enabled we could detect
repeated foreign key referencing from the same source.

On 15 Aug 2017 19:44, "Gordon Wrigley"  wrote:

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  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 
> 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
>>  
>> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Gordon Wrigley
In my current version each object keeps a reference to a WeakSet of the
results of the queryset it came from.
This is populated in _fetch_all and if it is populated then
ForwardManyToOneDescriptor does a prefetch across all the objects in the
WeakSet instead of it's regular fetching.

On Tue, Aug 15, 2017 at 8:03 PM, Collin Anderson 
wrote:

> Hi Gordon,
>
> How is it implemented? Does each object keep a reference to the queryset
> it came from?
>
> Collin
>
> On Tue, Aug 15, 2017 at 2:44 PM, Gordon Wrigley 
> wrote:
>
>> 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 
>> 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 
>>> 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
  
 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Collin Anderson
Hi Gordon,

How is it implemented? Does each object keep a reference to the queryset it
came from?

Collin

On Tue, Aug 15, 2017 at 2:44 PM, Gordon Wrigley 
wrote:

> 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 
> 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 
>> 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
>>>  
>>> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Gordon Wrigley
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  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 
> 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
>>  
>> 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 

Re: Automatic prefetching in querysets

2017-08-15 Thread Marc Tamlyn
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  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
>  
> 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 
> (which I was involved in creating) and 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