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

# Find Enum.map with Repo calls nearby
grep -B5 -A5 "Enum.map" lib/ -r --include="*.ex" | grep -A5 -B5 "Repo\."

# Find association access patterns
grep -r "\.posts\|\.comments\|\.orders" lib/ --include="*.ex"

# Find Repo calls in loops
grep -B3 "Repo.get\|Repo.one" lib/ -r --include="*.ex" | grep -B3 "for\|Enum"

Analysis Command

For a context module, run:

grep -n "Repo\." lib/my_app/[context].ex

Then verify each query has appropriate preloads.

References

For detailed patterns, see:

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