Django Rest Framework pagination extremely slow co

问题:

I turned pagination on in Django Rest framework and it appears to be incredibly slow. Count looks like the culprit, and is taking hundreds of milliseconds to return each time due to the millions of rows in the tables.

I am using postgresql as the database. Is there any way to not count the rows and still use pagination? The performance was fine before this was enabled if I manually filtered the queryset.

回答1:

Override the get_paginated_response method of your pagination class, and do not include the count. You can refer to the base implementation of the PageNumberPagination class to see what you should return.

from rest_framework.pagination import PageNumberPagination
from collections import OrderedDict # requires Python 2.7 or later

class PageNumberPaginationWithoutCount(PageNumberPagination):
    # Set any other options you want here like page_size

    def get_paginated_response(self, data):
        return Response(OrderedDict([
            ('next', self.get_next_link()),
            ('previous', self.get_previous_link()),
            ('results', data)
        ]))

Then in your settings.py, set DEFAULT_PAGINATION_CLASS to your new pagination class.

DEFAULT_PAGINATION_CLASS = 'path.to.PageNumberPaginationWithoutCount'

This approach is used in the example in the pagination docs.

Edit: from the comments below it sounds like this might not be enough to prevent the slow sql query, so you might need to override paginate_queryset as well.

回答2:

If you are ok without count, next and previous links, Following custom class can be used.

import sys
from collections import OrderedDict

from django.core.paginator import Paginator
from django.utils.functional import cached_property
from rest_framework.pagination import PageNumberPagination
from rest_framework.response import Response


class CustomPaginatorClass(Paginator):
    @cached_property
    def count(self):
        return sys.maxsize


# To Avoid large table count query, We can use this paginator class
class LargeTablePagination(PageNumberPagination):
    django_paginator_class = CustomPaginatorClass

    def get_paginated_response(self, data):
        return Response(OrderedDict([
            ('page', self.page.number),
            ('results', data)
        ]))

回答3:

The issue is, that the query used to count is the same potentially complex one as used to fetch the data. That's rather wasteful. PageNumberPagination uses Django's own Paginator internally.

To make the query for the count simpler override the paginator class DRF uses:

from django.core.paginator import Paginator
from django.utils.functional import cached_property
from rest_framework.pagination import PageNumberPagination

class FasterDjangoPaginator(Paginator):
    @cached_property
    def count(self):
        # only select 'id' for counting, much cheaper
        return self.object_list.values('id').count()


class FasterPageNumberPagination(PageNumberPagination):
    django_paginator_class = FasterDjangoPaginator