@ManyToMany relationship in Spring Boot - Performance & Optimization

Performance: @ManyToMany relationship

MEDIUM IMPACT

This affects database query performance and page load speed when fetching related data in web applications.

Fetching related entities in a @ManyToMany relationship

Spring Boot

@Entity
public class Student {
  @ManyToMany(fetch = FetchType.LAZY)
  private Set<Course> courses;
}

Lazy fetching loads related entities only when accessed, reducing initial query size and speeding up page load.

📈 Performance GainReduces initial database load and speeds up LCP by deferring related data fetching.

Fetching related entities in a @ManyToMany relationship

Spring Boot

@Entity
public class Student {
  @ManyToMany(fetch = FetchType.EAGER)
  private Set<Course> courses;
}

Eager fetching loads all related entities immediately, causing large joins or multiple queries that slow down page load.

📉 Performance CostBlocks rendering until all related data loads; can trigger multiple heavy database queries increasing LCP.

Performance Comparison

Pattern	Database Queries	Data Size	Page Load Impact	Verdict
Eager Fetching @ManyToMany	Multiple or large join queries	Large data fetched upfront	Slower LCP due to blocking	[X] Bad
Lazy Fetching @ManyToMany	Queries only when accessed	Smaller initial data size	Faster LCP by deferring load	[OK] Good

Rendering Pipeline

When a page requests data with @ManyToMany eager fetch, the backend queries all related entities before sending response, delaying HTML rendering.

→Data Fetching

→Backend Processing

→HTML Rendering

⚠️ BottleneckData Fetching due to large or multiple SQL queries caused by eager loading

Core Web Vital Affected

LCP

This affects database query performance and page load speed when fetching related data in web applications.

Optimization Tips

1Use FetchType.LAZY for @ManyToMany to avoid loading large related data sets upfront.

2Avoid accessing lazy collections in the view layer without pagination or limits.

3Consider DTO projections or custom queries to fetch only needed data.

Performance Quiz - 3 Questions

What is the main performance issue with using FetchType.EAGER on a @ManyToMany relationship?

A It loads all related entities immediately, causing slow database queries and page load.

B It delays loading related entities until accessed, causing UI lag.

C It caches related entities in the browser, increasing memory usage.

D It prevents any related entities from loading automatically.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and observe backend API calls and their response times.

What to look for: Look for large or multiple slow API calls caused by fetching many related entities upfront.