Django QuerySet Prefetching with Related Fields Causing Performance Issues in Large Datasets
I'm deploying to production and I'm attempting to set up Hey everyone, I'm running into an issue that's driving me crazy... I'm currently working on a Django application, and I'm facing a performance bottleneck when querying large datasets with related fields. I'm using Django 4.1 and have a model structure that includes a `User` model related to a `Profile` model and a `Post` model. Specifically, I'm trying to fetch all users along with their profiles and the posts they've made. Here’s a snippet of my code: ```python from django.db.models import Prefetch users_with_posts = User.objects.prefetch_related( 'profile', Prefetch('post_set', queryset=Post.objects.all().only('id', 'title')) ) ``` I expected this to optimize my queries, but when I run this in a view that handles about 10,000 users, I notice a significant slow down, with the response time increasing by several seconds. I’ve confirmed that the queries are indeed being executed, but I’m not sure where the bottleneck lies. When I inspect the generated SQL, I see that Django is performing multiple queries rather than a single optimized query. Additionally, I’m receiving the following warning in the console: ``` Warning: QuerySet is using a `Prefetch` object which may lead to N+1 queries if related fields are not properly configured. ``` I’ve tried tweaking the `select_related` method as well, but it doesn’t seem to help with this specific case since the relationships are one-to-many. Does anyone have insights on how to effectively prefetch related fields or manage performance issues with a large dataset in Django? Any suggestions or best practices would be appreciated! I'm working on a mobile app that needs to handle this. This issue appeared after updating to Python 3.9. Is this even possible? I'm working with Python in a Docker container on Windows 11. Has anyone else encountered this? Is there a better approach? Any suggestions would be helpful.