python 30 lines · 1 tab

Django query optimization with only and defer

Priya Sharma Jan 2026

1 tab

from blog.models import Post


# Load only specific fields
posts = Post.objects.only('id', 'title', 'published_at')
for post in posts:
    print(post.title)  # No extra query
    # print(post.content)  # Would trigger extra query


# Exclude large fields
posts = Post.objects.defer('content', 'raw_content')
for post in posts:
    print(post.title)  # No extra query
    # Accessing content would trigger query


# Combine with select_related
posts = Post.objects.select_related('author').only(
    'id', 'title', 'author__username'
)


# For API responses
def get_post_list_data():
    return Post.objects.only(
        'id', 'title', 'slug', 'published_at', 'author_id'
    ).select_related('author').only(
        'author__id', 'author__username'
    )

1 file · python Explain with highlit

The only() method loads specified fields only, while defer() excludes specified fields. This reduces data transfer and memory usage. I use only() when I need just a few fields from large models. For read-only displays, this prevents loading unnecessary data. I'm careful with deferred fields—accessing them causes additional queries. For JSON APIs, only() with serializer fields prevents over-fetching. I combine with select_related() for efficient joins. These optimizations are crucial for models with many or large fields.

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