N+1 Query Detection
Identify and fix N+1 query anti-patterns in Ecto/Phoenix applications.
Iron Laws - Never Violate These
- Never access associations without preload - Always preload before
Enum.map - No Repo calls inside loops - Restructure to batch queries
- Preload at context boundary - Load associations in context, not controllers/views
- Use joins for filtering - Use
join+preloadwhen filtering by association
Detection Patterns
Pattern 1: Enum.map with Repo
elixir# BAD: N+1 queries users |> Enum.map(fn user -> Repo.get(Order, user.order_id) end) # GOOD: Single query with preload users |> Repo.preload(:orders)
Pattern 2: Association Access Without Preload
elixir# BAD: Lazy loading triggers N queries for user <- users do user.posts # Triggers query for each user! end # GOOD: Eager load first users = Repo.all(User) |> Repo.preload(:posts) for user <- users do user.posts # Already loaded end
Pattern 3: Nested Association Access
elixir# BAD: N+1 for nested associations user.posts |> Enum.map(fn post -> post.comments end) # GOOD: Nested preload Repo.preload(user, posts: :comments)
Quick Detection Commands
Use Grep with context lines (-B 5 -A 5) to find Enum.map near Repo. calls in lib/**/*.ex.
Use Grep to find association access patterns (.posts, .comments, .orders) in lib/**/*.ex.
Use Grep with context (-B 3) to find Repo.get or Repo.one near loop patterns (for, Enum) in lib/**/*.ex.
Analysis Command
Use Grep to find all Repo. calls in a context module, then verify each query has appropriate preloads.
References
For detailed patterns, see:
references/preload-patterns.md- Efficient preloading strategiesreferences/query-optimization.md- Query batching techniques
