Fetch types (LAZY vs EAGER) in Spring Boot - Performance Comparison

Performance: Fetch types (LAZY vs EAGER)

HIGH IMPACT

This affects how and when related data is loaded from the database, impacting page load speed and responsiveness.

Loading related entities in a Spring Boot application

Spring Boot

class User {
  @OneToMany(fetch = FetchType.LAZY)
  Set<Order> orders;
}

// Orders load only when accessed explicitly

LAZY fetch delays loading related data until accessed, reducing initial query size and speeding up page load.

📈 Performance GainReduces initial DB load and memory use; improves LCP by loading only needed data.

Loading related entities in a Spring Boot application

Spring Boot

class User {
  @OneToMany(fetch = FetchType.EAGER)
  Set<Order> orders;
}

// Fetching User loads all Orders immediately

EAGER fetch loads all related entities immediately, even if not needed, causing slower initial queries and higher memory use.

📉 Performance CostBlocks rendering longer due to heavy DB queries; increases LCP by loading unnecessary data upfront.

Performance Comparison

Pattern	DB Queries	Memory Usage	Initial Load Time	Verdict
EAGER Fetch	Loads all related data immediately	High due to all data in memory	Slower initial load	[X] Bad
LAZY Fetch	Loads related data on demand	Lower memory footprint	Faster initial load	[OK] Good

Rendering Pipeline

Fetch type choice affects when data is retrieved from the database, influencing the backend response time and how fast the frontend can render content.

→Data Fetching

→Backend Processing

→Frontend Rendering

⚠️ BottleneckData Fetching stage due to large or unnecessary queries with EAGER fetch.

Core Web Vital Affected

LCP

This affects how and when related data is loaded from the database, impacting page load speed and responsiveness.

Optimization Tips

1Use LAZY fetch to load related data only when needed.

2Avoid EAGER fetch for large or deeply nested relationships.

3Monitor backend response times to detect heavy eager loading.

Performance Quiz - 3 Questions

What is the main performance drawback of using EAGER fetch in Spring Boot?

A It delays loading related data until accessed.

B It loads all related data immediately, increasing initial load time.

C It reduces memory usage by loading less data.

D It improves backend response time by batching queries.

DevTools: Network and Performance panels

How to check: Use Network panel to observe backend API response times; use Performance panel to see how long rendering waits for data.

What to look for: Long backend response times or large payloads indicate EAGER fetch; faster responses with smaller payloads indicate LAZY fetch.