The Scenario

Your admin dashboard takes 8 seconds to load the user list. Each page shows 100 users with their profile details.

The logs show 101 database queries:

1. SELECT user_id FROM users LIMIT 100
2. SELECT * FROM profiles WHERE user_id=1
3. SELECT * FROM profiles WHERE user_id=2
4. ... (98 more queries)

This is the N+1 Query Problem - the silent killer of application performance.

The Problem

Your code fetches the user list, then loops through users calling get_profile(user_id). Each call makes a separate database query. With 100 users, that's 100 queries.

If each query takes 50ms (typical for remote databases), that's 5 seconds just waiting on I/O.

The Goal

Refactor to use batch fetching - fetch all profiles in a single query.

Requirements:

• Fetch all needed profiles in ONE query using WHERE user_id IN (...)
• Total execution time must be < 100ms (with mocking)
• Must return the same data structure as before