【连载】关系型数据库是如何工作的？（9）
查询管理器 查询管理是一个数据库强大与否的一个判断指标。通过查询管理，可以把一个糟糕的查询语句转换为一段快速执行的代码，代码执行后返回结果给客户端管理器。整个过程分为多步： 查询首先被解析并检查其有效性； 重写查询并删除不必要的操作，另外做一
查询管理器
查询管理是一个数据库强大与否的一个判断指标。通过查询管理，可以把一个糟糕的查询语句转换为一段快速执行的代码，代码执行后返回结果给客户端管理器。整个过程分为多步：
- 查询首先被解析并检查其有效性；
- 重写查询并删除不必要的操作，另外做一些预优化；
- 为了提升性能进行必要的优化，最终转换为一个执行计划；
- 编译执行计划；
- 最后运行执行计划。
在读了这一章节之后，如果对查询优化想更深入的理解，我推荐阅读以下资料：
- 1979年发表的一篇最早的关于基于成本优化的论文： Access Path Selection in a Relational Database Management System。这篇论文只有12页，只要具备计算机科学中级水平就可以理解。
- 关于DB 9.X如何优化查询的深度介绍：DB2 for Linux, UNIX, and Windows Optimizer。
- 一篇关于PostgreSQL如何优化查询的详细介绍：Explaining the Postgres Query Optimizer。这篇非常易于理解，因为它并不是从PostgreSQL的算法角度介绍，而是展示了PostgreSQL的查询计划。
- SQLite官方优化文档：The SQLite Query Planner。它很容易读懂，以你为SQLite优化规则很简单；但是它仅仅解释了优化器如何工作。
- SQL Server 2005优化查询的介绍：Understanding Query Processing and Query Plans in SQL Server
- Orcale 12c优化白皮书：Optimizer with Oracle Database 12c
- 《DATABASE SYSTEM CONCEPTS》的作者两篇关于查询优化的理论课程，它们主要聚焦于磁盘IO：Chapter 12: Query Processing、Chapter 13: Query Optimization
- 还有一篇我找到的更容易理解的只介绍join操作和磁盘IO的理论课程：Datenbanksysteme II: Implementation of Database Systems
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