How can I improve the speed and performance of database queries?

I'm currently working on a Django project where I'm encountering performance issues, especially as my application scales. To provide better context, I've included a realistic code snippet below that represents a performance-intensive section of my application:

from django.db import models

class Order(models.Model):
    # Fields and relationships...

class OrderItem(models.Model):
    order = models.ForeignKey(Order, on_delete=models.CASCADE)
    product = models.ForeignKey('Product', on_delete=models.CASCADE)
    quantity = models.PositiveIntegerField()

class Product(models.Model):
    # Fields and relationships...

def calculate_total_price(order_id):
    order_items = OrderItem.objects.filter(order_id=order_id).select_related('product')
    total_price = 0

    for item in order_items:
        product_price = item.product.price
        quantity = item.quantity
        total_price += product_price * quantity

    return total_price

In the above code snippet, I have an Order model that is associated with multiple OrderItems, and each OrderItem references a Product. The calculate_total_price function is used to calculate the total price for a given order by iterating over its associated OrderItems. However, as the number of OrderItems increases, the calculation becomes significantly slower.

I would greatly appreciate any insights and suggestions on how to optimize this code snippet or any other general Django performance tips. Are there any specific techniques, database query optimizations, caching mechanisms, or Django features I should consider to enhance the performance of this code? Additionally, if there are any profiling tools or libraries you recommend for identifying and resolving performance issues, please let me know.

I'm keen to learn best practices and advanced techniques for optimizing Django performance, as I want to ensure my application performs efficiently as it grows. Thank you in advance for your valuable assistance!

I'm not sure what to do about the performance issues so I don't know where to begin optimisations.

Edit
Including example view code:

class OrderDetailView(View):
    template_name = 'order_detail.html'

    def get(self, request, order_id):
        order = Order.objects.get(id=order_id)
        total_price = calculate_total_price(order_id)
        context = {
            'order': order,
            'total_price': total_price
        }
        return render(request, self.template_name, context)

You can calculate the total price of the order using aggregate rather than looping over each item.

In your case, you can rewrite the function as follows:

from django.db.models import Sum, F

def calculate_total_price(order_id):
    total_price_obj = OrderItem.objects.filter(order_id=order_id).aggregate(total_price=Sum(F('item__product__price') * F('item__quantity')))
    total_price = total_price_obj.get('total_price') or 0
    return total_price

Here I am using or 0 while fetching the total price to set the total price = 0 if the queryset is empty. Without using it you'll get None as total price which might throw an exception in your codebase later on.

You can calculate this by the database itself, even in the same query as the one where you fetch the order with:

<pre><code>from django.db.models import F, Sum, Value
from django.db.models.functions import Coalesce
from django.shortcuts import get_object_or_404

class OrderDetailView(View):
template_name = 'order_detail.html'

def get(self, request, order_id):
    order = get_object_or_404(
        Order.objects.annotate(
            total_price=Coalesce(
                <b>Sum(
                    F('orderitem__quantity') * F('orderitem__product__price')
                )</b>,
                Value(0),
            )
        ),
        id=order_id,
    )
    context = {
        'order': order,
    }
    return render(request, self.template_name, context)</code></pre>

in the template you then render this with {{ order.total_price }}.

> Note: It is often better to use <code>get_object_or_404(&hellip;)</code>&nbsp;^[Django-doc],
> then to use <code>.get(&hellip;)</code>&nbsp;^[Django-doc] directly. In case the object does not exists,
> for example because the user altered the URL themselves, the <code>get_object_or_404(&hellip;)</code> will result in returning a HTTP 404 Not Found response, whereas using
> <code>.get(&hellip;)</code> will result in a HTTP 500 Server Error.

> Note: Subclassing the View class directly is often not necessary. In this case your view looks like a DetailView&nbsp;^[Django-doc]. By using a DetailView instead of a simple View, you often do not have to implement a lot of boilerplate code.

Switch from SQLite to PostgreSQL as your database to improved performance and efficiency