63 lines
1.6 KiB
Markdown
63 lines
1.6 KiB
Markdown
---
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title: "N+1 Query Prevention"
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type: concept
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tags: [database, performance, n+1, postgresql, query-optimization]
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last_updated: 2026-05-01
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---
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# N+1 Query Prevention
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## Definition
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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。
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## The Problem
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```typescript
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// ❌ Bad: N+1 pattern
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const users = await db.query("SELECT * FROM users LIMIT 10");
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for (const user of users) {
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user.posts = await db.query(
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"SELECT * FROM posts WHERE user_id = $1",
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[user.id]
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);
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}
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// 1 + 10 = 11 queries
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```
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## The Solution
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```typescript
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// ✅ Good: Single query with aggregation
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const usersWithPosts = await db.query(`
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SELECT
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u.id, u.email, u.name,
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COALESCE(
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json_agg(
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json_build_object('id', p.id, 'title', p.title)
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) FILTER (WHERE p.id IS NOT NULL),
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'[]'
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) as posts
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FROM users u
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LEFT JOIN posts p ON p.user_id = u.id
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GROUP BY u.id
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LIMIT 10
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`);
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// 1 query
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```
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## Key Techniques
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- **JOIN + json_agg**：PostgreSQL 下用 `json_agg` 聚合嵌套关联数据
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- **批量查询（Batch Loading）**：拆分为 2-3 次查询（IN 子句分批）
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- **预加载（Eager Loading）**：ORM 的 `include` / `preload` 机制
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- **物化视图**：对稳定结构做预计算
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## Source
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- [[engineering-database-optimizer]]
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## Connections
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- [[QueryPlanAnalysis]] — 通过 EXPLAIN ANALYZE 识别 N+1
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- [[engineering-backend-architect]] — 架构设计时需避免 N+1
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- [[engineering-sre]] — N+1 是生产环境性能问题的常见根源
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