November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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134
第 7 章
聚合框架
许多应用程序需要进行某一类的数据分析。
MongoDB
为使用聚合框架原生地进行分析提
供了强大的支持。本章介绍这个框架和它提供的一些基本工具。
•
聚合框架
•
聚合阶段
•
聚合表达式
•
聚合累加器
第
8
章会深入研究更高级的聚合特性，包括跨集合执行连接的能力。
7.1
管道
、
阶段和可调参数
聚合框架是
MongoDB
中的一组分析工具，可以对一个或多个集合中的文档进行分析。
聚合框架基于管道的概念。使用聚合管道可以从
MongoDB
集合获取
输入，并将该集合中的
文档传递到一个或多个阶段，每个阶段对其输入执行不同的操作（参见图
7-1
）
。每个阶段
都将之前阶段输出的内容作为输入。所有阶段的输入和输出都是文档——可以称为文档流。
集合
阶段 输出
...
管道
图 7-1：聚合管道
聚合框架
｜
135
如果你熟悉
Linux shell
中的管道，比如
bash
，那么这是一个非常相似的概念。每个阶段都
有其特定的工作。它会接收特定形式的文档并产生特定的输出，该输出本身就是文档流。
可以在管道的终点对输出进行访问，这与执行
find
查询的方式非常相似。也就是说，我们
获取一个文档流，然后对其做一些处理，无论是创建某种类型的报告、生成一个网站，还
是其他类型的任务。
现在来更深入地研究各个阶段。在聚合管道中，一个阶段就是一个数据处理单元。它一次接 ...
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