N+1 Query Detection skill

N+1 Query Detection

Identify and fix N+1 query anti-patterns in Ecto/Phoenix applications.

Iron Laws - Never Violate These

1. Never access associations without preload - Always preload before Enum.map
2. No Repo calls inside loops - Restructure to batch queries
3. Preload at context boundary - Load associations in context, not controllers/views
4. Use joins for filtering - Use join + preload when filtering by association

Detection Patterns

Pattern 1: Enum.map with Repo

# 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

# 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

# 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:

• ${CLAUDE_SKILL_DIR}/references/preload-patterns.md - Efficient preloading strategies
• ${CLAUDE_SKILL_DIR}/references/query-optimization.md - Query batching techniques