What is the N+1 query problem specific to GraphQL, and how does DataLoader solve it?

Because a GraphQL query can request nested related data (like a list of posts, each with their author), a naive resolver implementation ends up calling the database once per post to fetch its author individually, exactly the N+1 problem -- DataLoader solves this by batching and caching individual load requests that occur within the same tick of the event loop, automatically combining what would have been N separate database calls into a single batched query, while still letting each resolver be written as if it were making an independent request.

const DataLoader = require('dataloader');

const userLoader = new DataLoader(async (userIds) => {
  const users = await db.users.findByIds(userIds); // one batched query for all requested IDs
  return userIds.map(id => users.find(u => u.id === id));
});

// Each resolver call looks independent, but DataLoader batches them automatically
const resolvers = {
  Post: { author: (post) => userLoader.load(post.authorId) },
};