[Django Index] Improper Index Use Issue and Examples

Maintaining efficient query performance is difficult and it requires a proper indexing in Django models. Failing to define indexes on frequently queried fields can lead to significant performance degradation. Here are some common problems and their solutions.

Problem 1: Slow Query Performance

Scenario: A Django application has a Staff model with a first_name field frequently used in queries, but no index is defined.

Solution: Add an index to the username field.

from django.db import models

class Staff(models.Model): first_name = models.CharField(max_length=255, db_index=True)

Problem 2: Inefficient Filtering

Scenario: Filtering a large BlogPost table by the published_date field results in slow query times.

Solution: Create an index on the published_date field.

NOTE: A side effect of this solution is slow inserting/updating queries.

from django.db import models

class BlogPost(models.Model): published_date = models.DateTimeField(db_index=True)

Problem 3: Slow Foreign Key Lookups

Scenario: ForeignKey queries are slow due to some NULL values.

Solution: By default, Django creates index for ForeignKey field. However, for some cases like NULLable ForeignKey, we should do the opposite way.

Disable default index
Create a new index with condition.

class Author(models.Model):
    first_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)


class Book(models.Model):
    name = models.CharField(max_length=255)
    author = models.ForeignKey(
        "Author", on_delete=models.CASCADE, null=True, blank=True, db_index=False
    )

    class Meta:
        indexes = [
            models.Index(
                fields=["author"],
                condition=~models.Q(author=None),
            ),
        ]

Conclusion

By addressing these issues and ensuring proper indexing, you can significantly improve the performance of your Django application. Proper indexing reduces query times, enhances user experience, and optimizes overall application efficiency.

Find out more about Django Query Optimization: