N+1 query problem in APIs and how to fix it with Prisma

The N+1 Query Problem

The N+1 problem is one of the most common API performance bugs. It happens when you fetch a list of N items and then make a separate database query for each item's related data — N+1 total queries.

The Problem

// BAD — N+1: 1 query for posts + 1 query per post for author
const posts = await prisma.post.findMany();
for (const post of posts) {
  post.author = await prisma.user.findUnique({ where: { id: post.authorId } });
  // This fires a separate SQL query for EVERY post
}

The Fix — Eager Loading with include

// GOOD — 1 query (or 2 with Prisma's batching)
const posts = await prisma.post.findMany({
  include: {
    author: {
      select: { id: true, name: true } // only fields you need
    }
  }
});
// posts[0].author.name is available — no extra queries

Checking Query Count

// Enable query logging during development
const prisma = new PrismaClient({
  log: ['query'] // prints every SQL statement to stdout
});

Enable query logging in development and count the number of queries per request. If a list endpoint fires more than 2-3 queries, investigate. With 100 posts, an N+1 bug turns one endpoint into 101 database calls — a 100x performance hit.