How is query optimization achieved when using select_related and prefetch_related methods in Django?

In Django, the methods select_related and prefetch_related are used to optimize database queries and reduce the number of database hits.

• select_related performs an SQL JOIN and loads related objects in a single query. This is efficient for ForeignKey and OneToOne relationships.

• prefetch_related executes separate queries for related objects and then links them in Python. This is useful for ManyToMany and reverse relationships.

Example:

# Without optimization
books = Book.objects.all()
for book in books:
    print(book.author.name)  # Each time a separate query to the author

# With select_related
books = Book.objects.select_related('author').all()
for book in books:
    print(book.author.name)  # One query with JOIN

# With prefetch_related
books = Book.objects.prefetch_related('genres').all()
for book in books:
    for genre in book.genres.all():
        print(genre.name)  # Two queries: books and genres

Thus, these methods reduce the number of queries and improve application performance.