Would that actually end up executing the same function twice?

I didn't state it on the original question, but the biggest issue is that 
on my use case, the annotation step is actually rather complicated and such 
wrapped in a method on the model, and then it's up to the external code to 
filter and sort by the annotated value. Having to use the expression every 
single time it's needed would defeat the purpose of it.

I agree though that having more methods on the queryset is bad, I would 
rather improve the annotation logic to be able to handle these cases, but 
might also be difficult.


El sábado, 10 de marzo de 2018, 8:51:32 (UTC-3), Josh Smeaton escribió:
>
> Sure - but you can always save the expression to a variable and use it 
> multiple times.
>
> mycalc = MyFunc('a', 'b')
> Model.objects.filter(GreaterEqual(mycalc, 0.6)).order_by(mycalc)
>
> I think we already have the building blocks we need to avoid adding 
> another queryset method.
>
> On Saturday, 10 March 2018 14:01:41 UTC+11, Cristiano Coelho wrote:
>>
>> It wouldn't work if you also want to order by the annotated value.
>>
>> El viernes, 9 de marzo de 2018, 8:27:36 (UTC-3), Josh Smeaton escribió:
>>>
>>> Would teaching filter() and friends to use expressions directly solve 
>>> your issue? You suggested using `alias` upthread, but that's only really 
>>> required so you can refer to it later? Unless you wanted to refer to the 
>>> field more than once, having each queryset method respect expressions 
>>> should be enough I think.
>>>
>>> https://github.com/django/django/pull/8119 adds boolean expression 
>>> support to filter. I believe most other queryset methods have support for 
>>> expressions now (order_by, values/values_list).
>>>
>>> For the alias used multiple times case, it should be enough to annotate 
>>> and then restrict with values if you don't actually want it in the 
>>> select/group list.
>>>
>>> On Friday, 9 March 2018 00:22:00 UTC+11, Cristiano Coelho wrote:
>>>>
>>>> The workaround, although extremely ugly and which will probably cause 
>>>> issues in the future (reason I only used it for the model I needed to do 
>>>> those odd queries) was to use a custom queryset/manager. Something like 
>>>> this.
>>>>
>>>> class FasterCountQuerySet(QuerySet):
>>>>     def count(self):
>>>>         return super(FasterCountQuerySet, self.values('pk')).count()
>>>> FasterCountManager = Manager.from_queryset(FasterCountQuerySet)
>>>>
>>>> But again, this is extremely ugly and will still cause a subquery, but 
>>>> without the unnecessary group by and extra function calls.
>>>>
>>>>
>>>> El miércoles, 7 de marzo de 2018, 19:48:01 (UTC-3), Jared Proffitt 
>>>> escribió:
>>>>>
>>>>> I have also run into this exact problem. Would love to get this fixed. 
>>>>> Have you found a good workaround?
>>>>>
>>>>> On Tuesday, December 26, 2017 at 12:37:16 PM UTC-6, Cristiano Coelho 
>>>>> wrote:
>>>>>>
>>>>>> Hello, I'm having a hard time explaining the exact issue but I hope 
>>>>>> it's clear enough.
>>>>>>
>>>>>>
>>>>>> Following this issue (
>>>>>> https://groups.google.com/forum/#!searchin/django-users/cristiano%7Csort:date/django-users/q6XdfyK29HA/TcE8oFitBQAJ)
>>>>>>  
>>>>>> from django users and a related ticket (
>>>>>> https://code.djangoproject.com/ticket/27719) that seems to be left 
>>>>>> out or forgotten already.
>>>>>>
>>>>>> There has to be a way to alias or annotate a value given an 
>>>>>> expression or SQL Function that doesn't necessarily aggregates data but 
>>>>>> rather work on a single value.
>>>>>>
>>>>>> Right now as shown on the django-users post, using annotate for this 
>>>>>> purpose will cause unexpected grouping and sub querying that could 
>>>>>> result 
>>>>>> in very slow and hard to debug queries.
>>>>>>
>>>>>> The core issue is that using annotate without a previous call either 
>>>>>> vaues or values_list, will work as expected, simply annotating a value 
>>>>>> and 
>>>>>> returning it as an additional column, but if an aggregate is added 
>>>>>> afterwards (such as count), the final query ends up being a redundant 
>>>>>> query 
>>>>>> where the annotated value is added to a group by clause (group by id + 
>>>>>> column), to a column as part of the select (function called twice) and 
>>>>>> then 
>>>>>> wrapped into a select * (subquery), which makes the extra column as part 
>>>>>> of 
>>>>>> the select and group by useless, unless the query had any kind of 
>>>>>> left/inner join in which case the group by might make sense (although 
>>>>>> not 
>>>>>> sure about the column showing up on the select clause)
>>>>>>
>>>>>> The ugly work around is to simply add a .values('id') at the end so 
>>>>>> the annotated value doesn't show on the group by and select sections, 
>>>>>> although the nested query still happens.
>>>>>>
>>>>>>
>>>>>> For this reason, there's currently no way to achieve the above 
>>>>>> without ugly work arounds or unnecessary database performance hits.
>>>>>>
>>>>>> The easiest option I believe would be to follow the ticket in order 
>>>>>> to implement an alias call that works exactly like annotate but doesn't 
>>>>>> trigger any grouping.
>>>>>>
>>>>>> A more complicated option is probably trying to make 
>>>>>> annotate/aggregate smarter, so all the unnecessary grouping and sub 
>>>>>> querying doesn't happen unless needed, for example, if the queryset 
>>>>>> didn't 
>>>>>> call values/values_list or if there are no relationships/joins used.
>>>>>>
>>>>>>
>>>>>> Example/demostration:
>>>>>>
>>>>>> Given the following queryset
>>>>>>
>>>>>> query1 = MyModel.objects.annotate(x=MyFunction('a', 'b')).filter(
>>>>>> x__gte=0.6).order_by('-x')
>>>>>>
>>>>>>
>>>>>> query1 SQL is good and looks like:
>>>>>>
>>>>>> SELECT id, a, b, myfunction(a, b) as x
>>>>>> FROM mymodel
>>>>>> WHERE myfunction(a, b) >= 0.6
>>>>>> ORDER BY x desc
>>>>>>
>>>>>> Notice how there's no group by, the ORM was smart enough to not 
>>>>>> include it since there was no previous call to values/values_list
>>>>>>
>>>>>>
>>>>>> If we run query1.count() the final SQL looks like:
>>>>>>
>>>>>> SELECT COUNT(*) FROM (
>>>>>>     SELECT id, myfunction(a, b) as x
>>>>>>     FROM mymodel
>>>>>>     WHERE myfunction(a ,b) >= 0.6
>>>>>>     GROUP BY id, myfunction(a ,b)
>>>>>> ) subquery
>>>>>>
>>>>>> which if myfunction is slow, will add a massive slow down that's not 
>>>>>> even needed, and should actually be just:
>>>>>>
>>>>>> SELECT count(*)
>>>>>> FROM mymodel
>>>>>> WHERE myfunction(a ,b) >= 0.6
>>>>>>
>>>>>>
>>>>>> while the other query should ONLY happen if the group by makes sense 
>>>>>> (i.e, if there's a join somewhere, or a values/values_list was used 
>>>>>> previously so id is not part of the group by statement)
>>>>>>
>>>>>> but if we work around the issue adding a query1.values('id').count(), 
>>>>>> the final query ends up better:
>>>>>>
>>>>>> SELECT COUNT(*) FROM (
>>>>>>     SELECT id
>>>>>>     FROM mymodel
>>>>>>     WHERE myfunction(a ,b) >= 0.6
>>>>>> ) subquery
>>>>>>
>>>>>>
>>>>>> I hope I could explain this clear enough with the example, and note 
>>>>>> that using a custom lookup is not possible since the value is required 
>>>>>> for 
>>>>>> the order_by to work.
>>>>>>
>>>>>>
>>>>>>

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