N+1 avoidance with DataLoader (GraphQL)

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                  import DataLoader from 'dataloader';
import { withClient } from '../../db/pool';

export function createUserLoader() {
  return new DataLoader<string, any>(async (ids) => {
    const rows = await withClient(async (client) => {
      const res = await client.query('SELECT id, name FROM users WHERE id = ANY($1)', [ids]);
      return res.rows;
    });

    const map = new Map(rows.map((r: any) => [String(r.id), r]));
    return ids.map((id) => map.get(id) ?? null);
  });
}

GraphQL makes it very easy to accidentally create N+1 query explosions. DataLoader batches requests for the same resource during a single tick and gives you per-request caching. The trick is scoping: loaders must be per-request, not global singletons, otherwise user A can see cached data from user B. I keep the key format simple (usually a string id) and make sure the batch function returns results in the same order as the keys. This keeps GraphQL’s ergonomics without sacrificing DB performance. Even outside GraphQL, a loader-like approach helps any time you see repeated lookups inside a loop.

Mateo Rodriguez

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N+1 avoidance with DataLoader (GraphQL)

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