1---
2title: Avoid N+1 Query Problems
3impact: HIGH
4impactDescription: N+1 queries are one of the most common performance killers
5tags: database, n-plus-one, queries, performance
6---
7 
8## Avoid N+1 Query Problems
9 
10N+1 queries occur when you fetch a list of entities, then make an additional query for each entity to load related data. Use eager loading with `relations`, query builder joins, or DataLoader to batch queries efficiently.
11 
12**Incorrect (lazy loading in loops causes N+1):**
13 
14```typescript
15// Lazy loading in loops causes N+1
16@Injectable()
17export class OrdersService {
18  async getOrdersWithItems(userId: string): Promise<Order[]> {
19    const orders = await this.orderRepo.find({ where: { userId } });
20    // 1 query for orders
21 
22    for (const order of orders) {
23      // N additional queries - one per order!
24      order.items = await this.itemRepo.find({ where: { orderId: order.id } });
25    }
26 
27    return orders;
28  }
29}
30 
31// Accessing lazy relations without loading
32@Controller('users')
33export class UsersController {
34  @Get()
35  async findAll(): Promise<User[]> {
36    const users = await this.userRepo.find();
37    // If User.posts is lazy-loaded, serializing triggers N queries
38    return users; // Each user.posts access = 1 query
39  }
40}
41```
42 
43**Correct (use relations for eager loading):**
44 
45```typescript
46// Use relations option for eager loading
47@Injectable()
48export class OrdersService {
49  async getOrdersWithItems(userId: string): Promise<Order[]> {
50    // Single query with JOIN
51    return this.orderRepo.find({
52      where: { userId },
53      relations: ['items', 'items.product'],
54    });
55  }
56}
57 
58// Use QueryBuilder for complex joins
59@Injectable()
60export class UsersService {
61  async getUsersWithPostCounts(): Promise<UserWithPostCount[]> {
62    return this.userRepo
63      .createQueryBuilder('user')
64      .leftJoin('user.posts', 'post')
65      .select('user.id', 'id')
66      .addSelect('user.name', 'name')
67      .addSelect('COUNT(post.id)', 'postCount')
68      .groupBy('user.id')
69      .getRawMany();
70  }
71 
72  async getActiveUsersWithPosts(): Promise<User[]> {
73    return this.userRepo
74      .createQueryBuilder('user')
75      .leftJoinAndSelect('user.posts', 'post')
76      .leftJoinAndSelect('post.comments', 'comment')
77      .where('user.isActive = :active', { active: true })
78      .andWhere('post.status = :status', { status: 'published' })
79      .getMany();
80  }
81}
82 
83// Use find options for specific fields
84async getOrderSummaries(userId: string): Promise<OrderSummary[]> {
85  return this.orderRepo.find({
86    where: { userId },
87    relations: ['items'],
88    select: {
89      id: true,
90      total: true,
91      status: true,
92      items: {
93        id: true,
94        quantity: true,
95        price: true,
96      },
97    },
98  });
99}
100 
101// Use DataLoader for GraphQL to batch and cache queries
102import DataLoader from 'dataloader';
103 
104@Injectable({ scope: Scope.REQUEST })
105export class PostsLoader {
106  constructor(private postsService: PostsService) {}
107 
108  readonly batchPosts = new DataLoader<string, Post[]>(async (userIds) => {
109    // Single query for all users' posts
110    const posts = await this.postsService.findByUserIds([...userIds]);
111 
112    // Group by userId
113    const postsMap = new Map<string, Post[]>();
114    for (const post of posts) {
115      const userPosts = postsMap.get(post.userId) || [];
116      userPosts.push(post);
117      postsMap.set(post.userId, userPosts);
118    }
119 
120    // Return in same order as input
121    return userIds.map((id) => postsMap.get(id) || []);
122  });
123}
124 
125// In resolver
126@ResolveField()
127async posts(@Parent() user: User): Promise<Post[]> {
128  // DataLoader batches multiple calls into single query
129  return this.postsLoader.batchPosts.load(user.id);
130}
131 
132// Enable query logging in development to detect N+1
133TypeOrmModule.forRoot({
134  logging: ['query', 'error'],
135  logger: 'advanced-console',
136});
137```
138 
139Reference: [TypeORM Relations](https://typeorm.io/relations)
140