Query Optimization

Optimizing Django query performance is critical for building performant web applications. Django provides many tools and methods for optimizing database queries in its Database access optimization documentation. In this blog post, we will explore a collection of additional and essential tips I’ve compiled over the years to help you pinpoint and resolve your inefficient Django queries. Kill Long-Running Queries with a Statement Timeout PostgreSQL supports a statement_timeout parameter that allows you to set a maximum time limit per query. This is useful for preventing long-running queries from tying up precious resources and slowing down your application. My team at PixieBrix experienced an incident where a few long-running queries resulted in a full database outage. Setting a statement timeout in your Django settings can help prevent this from happening. ...

Introduction When building a Django application, one of the key challenges developers face is optimizing database query performance. Django provides two tools,select_related and prefetch_related, that reduce the number of database queries, and increase the performance of your application. This blog post explores the power of these two methods and how to combine them to maximize your application’s query performance. My team at PixieBrix implemented these techniques and have seen a significant improvement in query performance and customer satisfaction. ...

Sorting a Django queryset by a custom order is trickier than it looks, and choosing the wrong solution for your use case can cripple your application’s performance. For example, let’s take a simple Todo model: class Todo(models.Model): class Priority(models.TextChoices): HIGH = "HIGH", _("High") MEDIUM = "MEDIUM", _("Medium") LOW = "LOW", _("Low") title = models.CharField(max_length=255) priority = models.CharField(max_length=10, choices=Priority.choices, db_index=True) done = models.BooleanField(default=False) And a list of todos: Id Title Priority Done 1 Add linters Medium Yes 2 Fix bug High No 3 Increase test coverage Medium No 4 Refactor views Low No 5 Run tests in CI High Yes Our query set goal is to order these items from high to low priority. For priority levels with multiple items, we’ll also order by title to keep the results consistent. The expected result should be the id’s in the following order: [2, 5, 1, 3, 4] ...

In Django, a related object is a model instance used in the one-to-many or many-to-many context. For example, let’s look at the built-in User model, which has a many-to-many relationship with the Group model. class User(models.Model): groups = models.ManyToManyField(Group, related_name="groups") For any given User object, all linked Group objects are called “related objects”. Here are 5 ways to fetch all User objects with at least one related Group object. Iterate over each object in Python users = [] for user in User.objects.prefetch_related("groups"): if user.groups.exists(): users.append(user) Probably the most popular approach but there are two problems: ...

The Django ORM is a powerful tool but certain aspects of it are counterintuitive, such as the SQL order of execution. Let’s look at an example of this trap and how we can fix it using subqueries: class Book(models.Model): class Meta: constraints = [ models.UniqueConstraint( fields=["name", "edition"], name="%(app_label)s_%(class)s_unique_name_edition", ) ] name = models.CharField(max_length=255) edition = models.CharField(max_length=255) release_year = models.PositiveIntegerField(null=True) I want to write a query that reads: ...