---
title: "N+1 Query Prevention"
type: concept
tags: [database, performance, n+1, postgresql, query-optimization]
last_updated: 2026-05-01
---

# N+1 Query Prevention

## Definition

N+1 查询问题是指应用程序在获取一组主对象后，对每个对象分别发起一次额外查询来获取关联数据的反模式。假设获取 N 个用户，每个用户查一次帖子，总共产生 1 + N 次数据库往返。

## The Problem

```typescript
// ❌ Bad: N+1 pattern
const users = await db.query("SELECT * FROM users LIMIT 10");
for (const user of users) {
  user.posts = await db.query(
    "SELECT * FROM posts WHERE user_id = $1",
    [user.id]
  );
}
// 1 + 10 = 11 queries
```

## The Solution

```typescript
// ✅ Good: Single query with aggregation
const usersWithPosts = await db.query(`
  SELECT
    u.id, u.email, u.name,
    COALESCE(
      json_agg(
        json_build_object('id', p.id, 'title', p.title)
      ) FILTER (WHERE p.id IS NOT NULL),
      '[]'
    ) as posts
  FROM users u
  LEFT JOIN posts p ON p.user_id = u.id
  GROUP BY u.id
  LIMIT 10
`);
// 1 query
```

## Key Techniques

- **JOIN + json_agg**：PostgreSQL 下用 `json_agg` 聚合嵌套关联数据
- **批量查询（Batch Loading）**：拆分为 2-3 次查询（IN 子句分批）
- **预加载（Eager Loading）**：ORM 的 `include` / `preload` 机制
- **物化视图**：对稳定结构做预计算

## Source
- [[engineering-database-optimizer]]

## Connections
- [[QueryPlanAnalysis]] — 通过 EXPLAIN ANALYZE 识别 N+1
- [[engineering-backend-architect]] — 架构设计时需避免 N+1
- [[engineering-sre]] — N+1 是生产环境性能问题的常见根源