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wiki/concepts/N1QueryPrevention.md

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+---
+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