Optimize Performance

Identify and fix performance bottlenecks in Rails applications.

Quick Reference

Tool	Use
bullet	N+1 detection in development
rack-mini-profiler	Endpoint timing breakdown
EXPLAIN ANALYZE	Query plan analysis

HARD-GATE

NEVER optimize without a baseline measurement
ALWAYS write a regression spec before optimizing (query count assertion)
ALWAYS verify with EXPLAIN ANALYZE for database changes

NEVER write the report as "I applied includes(:author), then wrote a spec
to lock it in." The spec MUST be written and shown failing BEFORE the fix
appears in your output. Reordering for narrative flow fails the audit even
when the underlying work was correct.

Output Style

When completing a performance optimization, output MUST follow this seven-step report order. Each step must appear in output:

# Performance Optimization — [Description]

## 1. Baseline
<N> queries for <endpoint/action> — source: <log line / profiler output>

## 2. Bottleneck
<N+1 on association X / missing index on column Y> — tool: <bullet / rack-mini-profiler / EXPLAIN ANALYZE>

## 3. Regression Spec — RED
`make_database_queries(count: <N>)` at <path>:<line> — failure: expected <M>, got <N>

## 4. Fix
<path>:<line> — <includes(:association) / add_index / cache block>

## 5. Regression Spec — GREEN
Spec passes ✓ (<M> queries)

## 6. EXPLAIN ANALYZE
Before: Seq Scan, actual time=<X>ms → After: Index Scan, actual time=<Y>ms

## 7. Quantified Improvement
Queries: <N> → <M> | p95: <X>ms → <Y>ms

Language: English unless explicitly requested otherwise.

Extended Resources

Less-Obvious Optimization

# Use counter_cache to avoid COUNT queries in loops
# In migration: add_column :users, :posts_count, :integer, default: 0
# In Post model: belongs_to :user, counter_cache: true
user.posts_count  # no extra query

Regression Spec (Query Count Assertion)

RSpec.describe "Post index performance" do
  it "loads posts with authors in a fixed number of queries" do
    create_list(:post, 10, :with_author)

    expect do
      get posts_path
    end.to make_database_queries(count: 2) # 1 posts query + 1 authors query
  end
end

Use the db-query-matchers gem or a custom make_database_queries matcher.

Run directly in rails dbconsole (PostgreSQL) after applying an index or query change:

EXPLAIN ANALYZE
  SELECT posts.*, users.name
  FROM posts
  INNER JOIN users ON users.id = posts.author_id
  WHERE posts.published = true;

Reference Files and External Links

Load these files only when their specific content is needed:

• references/tools.md — Use when you need detailed Bullet, rack-mini-profiler, or EXPLAIN ANALYZE configuration and installation steps

External references:

• Active Record Querying
• rack-mini-profiler
• Bullet gem

Integration

Skill	When to chain
write-tests	For regression specs
review-migration	When adding an index