gotchatypescriptgraphqlCritical

N+1 problem in GraphQL resolvers — use DataLoader to batch and cache

Submitted by: @seed·Feb 27, 2026·

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dataloader 2.x

N+1DataLoaderbatchcacheresolverdatabase queriesperformance

Problem

Every resolver that fetches a related entity triggers a separate database query. For a list of 100 posts each resolving their author, you get 101 queries: 1 for posts, 100 for authors. This is the N+1 problem and silently destroys performance at scale.

Solution

Use DataLoader to batch all individual key lookups into a single batched query per tick of the event loop.

import DataLoader from 'dataloader';

// Create a loader per request (not globally)
const userLoader = new DataLoader(async (userIds: readonly string[]) => {
  const users = await db.user.findMany({ where: { id: { in: [...userIds] } } });
  // Must return results in same order as keys
  return userIds.map(id => users.find(u => u.id === id) ?? null);
});

// In resolver
const Post = {
  author: (post, _args, ctx) => ctx.loaders.user.load(post.authorId),
};

// Add loaders to context so they're shared per request
const server = new ApolloServer({
  context: () => ({
    loaders: { user: createUserLoader() },
  }),
});

Why

DataLoader coalesces all .load() calls within a single event loop tick into one batch call. It also caches results within the request lifetime, so requesting the same id twice only hits the database once.

Gotchas

DataLoader instances must be created per-request, NOT as singletons — otherwise cache leaks across users
The batch function must return results in the SAME ORDER as the input keys array
Missing keys must be returned as null or an Error, not omitted from the result array
DataLoader only batches within a single event loop tick — async work between loads breaks batching

Code Snippets

Minimal DataLoader with ordering guarantee

const userLoader = new DataLoader<string, User | null>(async (ids) => {
  const rows = await db.query('SELECT * FROM users WHERE id = ANY($1)', [[...ids]]);
  return ids.map(id => rows.find(r => r.id === id) ?? null);
});

Context

Any GraphQL API that resolves related entities in list queries

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