Eager loading (N+1 prevention) in Ruby on Rails - Performance & Optimization

Performance: Eager loading (N+1 prevention)

HIGH IMPACT

This concept affects database query efficiency and page load speed by reducing the number of queries needed to fetch related data.

Loading a list of posts with their authors

Ruby on Rails

posts = Post.includes(:author).all
posts.each do |post|
  puts post.author.name
end

Eager loading fetches posts and their authors in two queries regardless of number of posts, reducing database hits.

📈 Performance GainTriggers only 2 database queries, significantly reducing load time and server work.

Loading a list of posts with their authors

Ruby on Rails

posts = Post.all
posts.each do |post|
  puts post.author.name
end

This triggers one query for posts plus one query per post to fetch the author, causing many database hits (N+1 queries).

📉 Performance CostTriggers 1 + N database queries, increasing page load time and server load.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
N+1 queries (lazy loading)	Normal	Multiple due to delayed data	Higher due to slower data	[X] Bad
Eager loading with includes	Normal	Single or fewer	Lower due to faster data	[OK] Good

Rendering Pipeline

Eager loading reduces the number of database queries, which speeds up data retrieval before rendering. This leads to faster style calculation and layout since data is ready sooner.

→Data Fetching

→Layout

→Paint

⚠️ BottleneckData Fetching (database queries)

Core Web Vital Affected

LCP

This concept affects database query efficiency and page load speed by reducing the number of queries needed to fetch related data.

Optimization Tips

1Avoid N+1 queries by using eager loading with includes or preload.

2Batch related data fetching to reduce database round trips.

3Monitor database queries in DevTools Network tab to detect N+1 issues.

Performance Quiz - 3 Questions

What is the main performance problem caused by N+1 queries?

A Many small database queries increasing load time

B Too much CSS causing slow paint

C Large JavaScript bundles blocking rendering

D Excessive DOM nodes causing reflows

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by XHR or fetch, reload page, count number of API/database calls made.

What to look for: Look for multiple repeated queries fetching related data separately (bad) versus fewer batched queries (good).