What is the N+1 query problem in Django and how do select_related and prefetch_related solve it? Django में N+1 क्वेरी समस्या क्या है और select_related व prefetch_related इसे कैसे हल करते हैं?

The N+1 problem occurs when iterating over N objects and accessing a related object on each one triggers a separate query per iteration (because Django's ORM is lazy by default), resulting in N+1 total queries instead of one efficient join — a common and easy-to-miss performance bug in list views and templates.

select_related() solves this for forward foreign-key and one-to-one relationships by performing a SQL JOIN and fetching the related object in the same query. prefetch_related() solves it for reverse foreign-key and many-to-many relationships, where a JOIN would multiply rows, by running a second separate query and joining the results in Python instead.

orders = Order.objects.all()
for order in orders:
    print(order.customer.name)  # extra query every iteration

orders = Order.objects.select_related('customer')
customers = Customer.objects.prefetch_related('orders')