Users
Loading 20 records. Compare query count and execution time between the two approaches.
Problem
The loop calls u.reviews.count() for each user. The reviews relationship is a reverse ForeignKey — Django fires a SELECT COUNT(*) query per user. For N users, you get N extra COUNT queries. Result: N+1 queries.
User.objects.all()
# template calls u.reviews.count() per userFix
annotate(review_count=Count('reviews')) adds the count directly into the main SQL query via a COUNT() subquery. The result is available as a plain attribute — 1 query total.
User.objects.annotate(review_count=Count('reviews'))Unoptimized
21 queries
13.0 ms
N+1 detected: 21 queries for 20 rows
| Name | # Reviews | |
|---|---|---|
| [email protected] | Hermila Ochoa | 9 |
| [email protected] | Ty Boyer | 5 |
| [email protected] | David Vazquez | 9 |
| [email protected] | Lewis Calhoun | 6 |
| [email protected] | Refugio Suarez | 2 |
| [email protected] | Kerry Moses | 9 |
| [email protected] | Kandace Washington | 5 |
| [email protected] | Cathey Strickland | 9 |
| [email protected] | Cassaundra Whitaker | 3 |
| [email protected] | Rodney Raymond | 7 |
| [email protected] | Timothy Melendez | 1 |
| [email protected] | Dreama Christensen | 8 |
| [email protected] | Allen Holmes | 7 |
| [email protected] | Shawana Forbes | 10 |
| [email protected] | Romaine Juarez | 4 |
| [email protected] | Mariano Camacho | 7 |
| [email protected] | Leisha Walls | 3 |
| [email protected] | Nevada Reeves | 4 |
| [email protected] | Chuck Vinson | 2 |
| [email protected] | Genie Moore | 5 |
Optimized
1 queries
4.6 ms
21 queries reduced to 1
(13.0ms to 4.6ms)
| Name | # Reviews | |
|---|---|---|
| [email protected] | Hermila Ochoa | 9 |
| [email protected] | Ty Boyer | 5 |
| [email protected] | David Vazquez | 9 |
| [email protected] | Lewis Calhoun | 6 |
| [email protected] | Refugio Suarez | 2 |
| [email protected] | Kerry Moses | 9 |
| [email protected] | Kandace Washington | 5 |
| [email protected] | Cathey Strickland | 9 |
| [email protected] | Cassaundra Whitaker | 3 |
| [email protected] | Rodney Raymond | 7 |
| [email protected] | Timothy Melendez | 1 |
| [email protected] | Dreama Christensen | 8 |
| [email protected] | Allen Holmes | 7 |
| [email protected] | Shawana Forbes | 10 |
| [email protected] | Romaine Juarez | 4 |
| [email protected] | Mariano Camacho | 7 |
| [email protected] | Leisha Walls | 3 |
| [email protected] | Nevada Reeves | 4 |
| [email protected] | Chuck Vinson | 2 |
| [email protected] | Genie Moore | 5 |