Severe Performance Issues with Data Aggregation in Django ORM when Filtering on Related Models
After trying multiple solutions online, I still can't figure this out. Could someone explain I tried several approaches but none seem to work..... I've searched everywhere and can't find a clear answer. I tried several approaches but none seem to work... I'm experiencing significant performance degradation in my Django application when trying to aggregate data from related models. My setup involves Django 4.0 with PostgreSQL 14. The specific issue arises when I attempt to filter and annotate a queryset involving multiple related models, resulting in very slow responses (around 10 seconds) for what should be a straightforward query. Here’s a simplified version of my models: ```python class Author(models.Model): name = models.CharField(max_length=100) class Book(models.Model): title = models.CharField(max_length=200) author = models.ForeignKey(Author, on_delete=models.CASCADE) published_date = models.DateField() class Review(models.Model): book = models.ForeignKey(Book, on_delete=models.CASCADE) rating = models.IntegerField() created_at = models.DateTimeField(auto_now_add=True) ``` I need to obtain a list of authors along with the average rating of their books, but when I try to do this using the following queryset, it takes forever: ```python from django.db.models import Avg result = Author.objects.annotate(avg_rating=Avg('book__review__rating')).filter(avg_rating__isnull=False) ``` I've tried optimizing this query by adding indexes on `book` and `rating` fields, but the performance didn’t improve significantly. Additionally, I’ve experimented with using `select_related` and `prefetch_related`, but it still results in poor performance. Here is what I tried: ```python result = Author.objects.prefetch_related('book__review').annotate(avg_rating=Avg('book__review__rating')).filter(avg_rating__isnull=False) ``` Even after these optimizations, the response time remains around 10 seconds. I checked the query being generated and it seems to be hitting a Cartesian product due to the way Django constructs the SQL. The raw SQL it generates is: ```sql SELECT author.id, AVG(review.rating) AS avg_rating FROM author LEFT JOIN book ON author.id = book.author_id LEFT JOIN review ON book.id = review.book_id GROUP BY author.id; ``` This seems inefficient, especially if there are a lot of books and reviews. Can anyone suggest a better approach to optimize this aggregation, or maybe a different way to structure my query to avoid this performance hit? I'm working on a API that needs to handle this. This is part of a larger CLI tool I'm building. What am I doing wrong? I've been using Python for about a year now. Is there a simpler solution I'm overlooking? I recently upgraded to Python latest. Could this be a known issue?