47 lines
1.5 KiB
Markdown
47 lines
1.5 KiB
Markdown
---
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title: "Query Plan Analysis"
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type: concept
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tags: [database, postgresql, explain, performance, query-optimization]
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last_updated: 2026-05-01
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---
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# Query Plan Analysis
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## Definition
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查询计划分析是通过 `EXPLAIN ANALYZE` 命令解读数据库优化器生成的查询执行计划，识别性能瓶颈（如 Seq Scan 全表扫描、估算偏差大）并针对性优化的方法。
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## Key Scan Types
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| 扫描类型 | 评估 | 说明 |
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|---------|------|------|
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| **Seq Scan** | ⚠️ 差 | 全表扫描，通常意味着缺索引 |
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| **Index Scan** | ✅ 好 | 索引扫描，直接定位数据 |
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| **Bitmap Heap Scan** | 🟡 可接受 | 先索引定位再堆查，适合中等结果集 |
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| **Index Only Scan** | ✅ 最佳 | 完全在索引中完成，无需回表 |
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## What to Look For
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- **actual time vs estimated rows**：与估算差异大说明统计信息过时
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- **cost=... loops=...**：高 cost 或多次 loops 需优化
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- **Buffers: shared hit/read**：大量 read 说明缓存未命中
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- **Seq Scan on large tables**：大表全表扫描通常是问题信号
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## Example
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```sql
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EXPLAIN ANALYZE
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SELECT * FROM posts WHERE user_id = 123;
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-- Good: Index Scan
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-- Bad: Seq Scan on posts (cost=0.00..1234.56 rows=500 width=...)
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```
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## Source
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- [[engineering-database-optimizer]]
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## Connections
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- [[IndexingStrategies]] — 根据分析结果创建/调整索引
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- [[N1QueryPrevention]] — EXPLAIN 识别 N+1 查询
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- [[SafeMigrations]] — 迁移前后验证查询性能
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