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N+1 queries: await inside a loop

Nodemid

Практика →

Main had a simple stub that just returned an array of orders by id. A colleague sent a PR enriching them with user and product data. In tests (3 orders) it runs in 100 ms; in production the endpoint takes 8 seconds. Walk the diff and find every N+1.

PR: lesson.typescript +18 −04 проблем

1-import type { Order } from './types'

1+import type { Order, User } from './types'

22 import { db } from './db'

4-export async function getOrdersWithUsers(orderIds: string[]): Promise<Order[]> {

5- return db.orders.findMany({ where: { id: { in: orderIds } } })

4+export async function getOrdersWithUsers(orderIds: string[]): Promise<(Order & { user: User })[]> {

5+ const result: (Order & { user: User })[] = []

7+ for (const orderId of orderIds) {

8+ const order = await db.orders.findById(orderId)

9+ if (!order) continue

10+

11+ const user = await db.users.findById(order.userId)

12+ if (!user) continue

13+

14+ const itemsWithDetails = []

15+ for (const item of order.items) {

16+ const product = await db.products.findById(item.productId)

17+ itemsWithDetails.push({ ...item, product })

18+ }

19+

20+ result.push({ ...order, user, items: itemsWithDetails } as any)

21+ }

22+

23+ return result

624 }

Шаг 1 из 6

What is N+1

N+1 is the classic data-access problem: instead of 1 query to the DB, you make N+1, where N is the collection size. Typical example: fetched the list of orders (1 query), then a separate user query for every order (N queries). Total — N+1. Sign in code: `await` inside `for` or `forEach` over an array, where a related resource is fetched. Find every N+1 in the PR — there are three.