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.
Given the following queryset
query1 SQL is good and looks like:
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:
which if myfunction is slow, will add a massive slow down that's not even needed, and should actually be just:
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:
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.
You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group.
To unsubscribe from this group and stop receiving emails from it, send an email to [hidden email].
To post to this group, send email to [hidden email].
Visit this group at https://groups.google.com/group/django-developers.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/4e4dbcd9-9c49-468b-b633-ca27daf3fe69%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
|Free forum by Nabble||Edit this page|