select_related() returns a QuerySet that follows foreign-key relationships, selecting additional related-object data when it executes its query. It creates a single complex query which means later use of foreign-key relationships will not hit the database which leads to a performance boost.

Example:

We will use the following models to demonstrate how it works

class Person(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    birthdate = models.DateField()

    def __str__(self):
        return self.first_name

class Book(models.Model):
    name = models.CharField(max_length=100)
    author = models.ForeignKey(Person, on_delete=models.CASCADE)

    def __str__(self):
        return self.name

 

if you want to get the author from Book instance, there are two ways of doing this, the first way is shown below

book = Book.objects.get(pk=5) # Hits the database
author = book.author  # Hits the database again 

And here’s select_related lookup:

# Hits the database.
book = Book.objects.select_related('author').get(pk=5)

# Doesn't hit the database, because book.author has been prepopulated
# in the previous query.
author = book.author
 

Using the second example is more efficient because we don't have to query the database twice, this is why we say select_related is a performance booster.

Resources

QuerySet API reference

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