Rails Performance Patterns Skill

Automatically detects performance issues and suggests optimizations.

What This Skill Does

Automatic Detection:

• N+1 query problems (missing eager loading)
• Missing database indexes on foreign keys
• Inefficient query patterns
• Large result sets without pagination

When It Activates:

• Model files with associations modified
• Controller actions that query models
• Iteration over associations detected

Key Checks

1. N+1 Query Detection

Problem Pattern:

# app/controllers/posts_controller.rb
def index
  @posts = Post.all  # 1 query
  @posts.each do |post|
    puts post.author.name  # N queries (one per post)
  end
end

Skill Output:

⚠️  Performance: Potential N+1 query
Location: app/controllers/posts_controller.rb:15
Issue: Accessing 'author' association in loop without eager loading

Fix: Add includes to eager load:
@posts = Post.includes(:author).all

Solution:

def index
  @posts = Post.includes(:author).all  # 2 queries total
end

2. Missing Indexes

Checks:

• Foreign keys have indexes
• Commonly queried columns indexed
• Unique constraints have indexes

Example:

# db/migrate/xxx_create_posts.rb
create_table :posts do |t|
  t.references :user  # ✅ Auto-creates index
  t.string :slug      # ❌ Missing index if queried often
end

Skill Output:

⚠️  Performance: Missing index recommendation
Location: app/models/post.rb:5
Issue: slug column used in where clauses without index

Add migration:
add_index :posts, :slug, unique: true

3. Pagination Missing

Problem:

def index
  @products = Product.all  # ❌ Loads all 100k+ products
end

Skill Output:

⚠️  Performance: Large result set without pagination
Location: app/controllers/products_controller.rb:10
Issue: Loading all Product records (estimated 100k+ rows)

Recommendation: Add pagination
# Use kaminari or pagy
@products = Product.page(params[:page]).per(20)

4. Counter Cache Opportunities

Pattern:

# Without counter cache
@user.posts.count  # Runs COUNT(*) query every time

# With counter cache
@user.posts_count  # Reads from cached column

Skill Output:

ℹ️  Performance: Counter cache opportunity
Location: app/views/users/show.html.erb:12
Pattern: Frequently accessing post count

Add to migration:
add_column :users, :posts_count, :integer, default: 0

Update association:
belongs_to :user, counter_cache: true

Configuration

# .rails-performance.yml
n1_detection:
  enabled: true
  severity: warning

indexes:
  check_foreign_keys: true
  check_query_columns: true

pagination:
  warn_threshold: 1000
  require_for_large_tables: true

counter_cache:
  suggest_threshold: 3  # Suggest if accessed 3+ times

Monitoring Integration

Add instrumentation:

# config/initializers/query_monitoring.rb
ActiveSupport::Notifications.subscribe('sql.active_record') do |*args|
  event = ActiveSupport::Notifications::Event.new(*args)
  if event.duration > 100  # Log slow queries
    Rails.logger.warn("Slow query (#{event.duration}ms): #{event.payload[:sql]}")
  end
end

Common Optimizations

Eager Loading:

# N+1
Post.all.each { |p| p.author.name }

# Fixed: includes (left join)
Post.includes(:author).each { |p| p.author.name }

# Fixed: preload (separate queries)
Post.preload(:author).each { |p| p.author.name }

# Fixed: eager_load (always joins)
Post.eager_load(:author).each { |p| p.author.name }

Select Specific Columns:

# Loads all columns
Post.all

# Loads only needed columns
Post.select(:id, :title, :created_at)

Batch Processing:

# Loads all at once
Post.all.each { |p| process(p) }

# Loads in batches of 1000
Post.find_each(batch_size: 1000) { |p| process(p) }

References

• Rails Performance Guide: https://guides.rubyonrails.org/performance_testing.html
• Bullet Gem: https://github.com/flyerhzm/bullet
• Pattern Library: /patterns/caching-patterns.md

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This skill helps you build fast Rails applications from the start.