id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35637	Paginator executes additional SQL query when using QuerySet	amir-mahdih86		"When using Django's `Paginator` class with a `QuerySet`, it executes an additional SQL count query to determine the total count of items. This does not happen when a `list` is passed to the `Paginator`.

**Steps to Reproduce:**
1. Create a view that uses `Paginator` with a `QuerySet` object.
2. Compare the SQL queries executed with those executed when a `list` is used instead.

**Expected Behavior:**
The `Paginator` shouldn't execute an additional query to count a QuerySet's items while it can use `len()` to do that.

**Actual Behavior:**
An additional query is executed when a `QuerySet` is passed while it's possible to convert `QuerySet` to a `list` before passing it to `Paginator` with no difference in returned object but 1 less database query execution.

**Example Code:**

{{{
# This code is simplified to be smaller

# This bellow view executes 3 SQL queries
class PostList(APIView):
    def get(self, request):
        posts = Post.objects.filter(is_published=True)
        paginator = Paginator(posts, 10)
        posts = paginator.page(1)
        serializer = PostSerializer(posts, many=True)
        return Response(serializer.data)


# But this one executes 2 SQL queries
class PostList(APIView):
    def get(self, request):
        posts = list(Post.objects.filter(is_published=True))
        paginator = Paginator(posts, 10)
        posts = paginator.page(1)
        serializer = PostSerializer(posts, many=True)
        return Response(serializer.data)
}}}

**My Solution:**
A simple solution is to modify the `__init__` method of the `Paginator` class to convert `object_list` to a `list` object.
This is just a simple solution and it may be necessary to enhance the problem in a deeper layer.

Here is the modified version of that method:
{{{
    def __init__(
        self,
        object_list,
        per_page,
        orphans=0,
        allow_empty_first_page=True,
        error_messages=None,
    ):
        self.object_list = list(object_list)
        self._check_object_list_is_ordered()
        self.per_page = int(per_page)
        self.orphans = int(orphans)
        self.allow_empty_first_page = allow_empty_first_page
        self.error_messages = (
            self.default_error_messages
            if error_messages is None
            else self.default_error_messages | error_messages
        )
}}}
"	Bug	closed	Database layer (models, ORM)	5.0	Normal	invalid	Paginator, Queryset, Performance, SQL Queries	amir-mahdih86	Unreviewed	1	0	0	1	0	0