Hello,
I think there is potential room for improvement with how Django handles
count() queries, specifically relating to annotations. Currently any
annotations that are added to a queryset are included in the SQL statement
that is generated in the count() query, including all joins and SQL
function calls - I've personally been bitten by this with DRF and a query
with a particularly complex set of annotations where the count query took
longer than fetching the data itself, but was semantically equivalent to a
much faster, plain Model.objects.count() call.
I think in most cases annotations can be stripped, for example in the two
queries below the annotation is not filtered on so it can be safely removed
from the query without affecting the result:
one = Book.objects.annotate(Count('chapters')).count()
two = Book.objects.count()
I wanted to gather some feedback from the group as to whether this always
holds true: if an annotation is not filtered (or ordered on) it can always
be safely removed from a count query? I wouldn't be surprised if there is a
case where this isn't safe, but I cannot think of one.
I've made an initial merge request that removes annotations from querysets
that have no filter or ordering that depends on annotations here:
https://github.com/django/django/pull/8928. It seems like Django also has
all the information to optimize out annotations that are not directly or
indirectly used by filters, but I wanted to gather some feedback before I
attempted this.
Tom
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