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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