Strip unused annotations from count queries

[Tom Forbes]

The query below produces a SQL statement that includes the Count('chapters'), despite not not being used in any filter operations.

Book.objects.annotate(Count('chapters')).count()

It produces the same results as:

Book.objects.count()

Django could be more intelligent about what annotations to include in the query produced by queryset.count(), stripping out any annotations that are not referenced by filters, other annotations or ordering. This should speed up calls to count() with complex annotations.

There seems to be precedent for this: select_related calls are ignored with count() queries.

[Tom Forbes]

Same can be done for exists()

[Tom Forbes]

WIP PR: ​https://github.com/django/django/pull/8928/files

[Tim Graham]

The PR is still marked [WIP] and there are test failures.

[Reupen Shah]

I have also run into problems with QuerySet.count() being very slow on annotated query sets. Django uses a subquery for the count but injects a group by into the subquery. This typically causes the database server to deduplicate all matched rows using the group by columns which is, in general, extremely slow when there are a large number of matched rows.

For example, consider the following model:

class Person(models.Model):
    """Person model."""

    first_name = models.TextField()
    last_name = models.TextField()
    country = models.TextField(null=True, blank=True)

and query set:

from django.db.models.functions import Concat
from django.db.models import Value

queryset = Person.objects.annotate(full_name=Concat('first_name', Value(' '), 'last_name'))

queryset.count() generates the following query under PostgreSQL:

SELECT COUNT(*)
FROM
  (SELECT "support_person"."id" AS Col1,
          CONCAT("support_person"."first_name", CONCAT(' ', "support_person"."last_name")) AS "full_name"
   FROM "support_person"
   GROUP BY "support_person"."id",
            CONCAT("support_person"."first_name", CONCAT(' ', "support_person"."last_name"))) subquery

list(queryset) generates:

SELECT "support_person"."id",
       "support_person"."first_name",
       "support_person"."last_name",
       "support_person"."country",
       CONCAT("support_person"."first_name", CONCAT(' ', "support_person"."last_name")) AS "full_name"
FROM "support_person"

I am not entirely sure why the subquery for the count needs to be any different from the query used when the query set itself is evaluated. There are some relevant comments in the source code here: ​https://github.com/django/django/blob/5deb7a86e8b54d052a3b1dbed1ae7142d362b1c5/django/db/models/sql/query.py#L404-L414

This has all been tested under Django 2.1.7.

[Simon Charette]

This is somewhat related to #30158 where the compiler is not smart enough to determine if it can exclude subquery annotations from GROUP BY on aggregation.

[Reupen Shah]

The behaviour is actually a bit more bizarre than I thought.

With the same person model, here are four count variations and the generated queries:

1.

Person.objects.count()

SELECT COUNT(*) AS "__count" FROM "people_person"

2.

Person.objects.values('pk').count()

SELECT COUNT(*) AS "__count" FROM "people_person"

3.

Person.objects.annotate(full_name=Concat('first_name', Value(' '), 'last_name')).count()

SELECT COUNT(*) FROM (
    SELECT "people_person"."id" AS Col1, CONCAT("people_person"."first_name", CONCAT(' ', "people_person"."last_name")) AS "full_name"
    FROM "people_person"
    GROUP BY "people_person"."id", CONCAT("people_person"."first_name", CONCAT(' ', "people_person"."last_name"))
) subquery

4.

Person.objects.annotate(full_name=Concat('first_name', Value(' '), 'last_name')).values('pk').count()

SELECT COUNT(*) FROM (SELECT "people_person"."id" AS Col1 FROM "people_person") subquery

So there's a simple workaround for the case I gave in my last comment in calling .values('pk') before .count(). However, that's not much help if Django or other libraries are the ones calling .count().

On the plus side, I have a better understanding of what the problem is from looking at the Django source code.

[Reupen Shah]

Just a note to say that the behaviour I was describing was rectified in ​https://github.com/django/django/pull/11062.

The third case from above now executes the following SQL query:

SELECT COUNT(*) FROM (
    SELECT CONCAT("people_person"."first_name", CONCAT(' ', "people_person"."last_name")) AS "full_name" FROM "people_person"
) subquery

Although the original ticket was about eliminating unneeded annotations completely, the result above is good enough for me, as the group by has been eliminated which was the real performance killer.