Django makes it easy to write code, which performs bad, since it is automatically executing SQL statements when you are handling models. If you don’t know about the details behind that, you can easily end up executing thousands of queries.
def get_books_by_author():
books = Book.objects.all()
result = defaultdict(list)
for book in books:
author = book.author
title_and_author = '{} by {}'.format(
book.title,
author.name
)
result[book.library_id].append(title_and_author) return result
This will execute for each book an author query! Which means 1 query for books and n queries for n books!
books = Book.objects.all().select_related('author')
This will add “author” into the Book.objects.all() query and you just execute one query!
However, select_related only works for one-to-many and one-to-one. Many to one relations can be optimized with prefetch_related().
Continue reading “Django (SQL/”Model”) Performance Optimization”