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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63
第 5 章
索引
本章介绍
MongoDB
的索引。索引使你能够高效地执行查询。它们是应用程序开发的重要
组成部分，甚至对于某些类型的查询是必需的。本章主要内容如下。
•
什么是索引？为什么要使用索引？
•
如何选择要创建索引的字段？
•
如何强制使用索引？如何对索引的使用进行评估？
•
创建及删除索引的管理细节。
你会发现，为集合选择正确的索引对性能至关重要。
5.1
索引简介
数据库索引类似于图书索引。有了索引便不需要浏览整本书，而是可以采取一种快捷方式，
只查看一个有内容引用的有序列表。这使得
MongoDB
的查找速度提高了好几个数量级。
不使用索引的查询称为
集合扫描
，这意味着服务器端必须“浏览整本书”才能得到查询的
结果。这个过程基本上类似于在一本没有索引的书中寻找信息：从第
1
页开
始，通读整本
书。通常来说，我们希望避免让服务器端执行集合扫描，因为对于大集合来说，该过程非
常缓慢。
来看一个例子。首先，创建一个包含
1 000 000
（或者
10 000 000
甚至
100 000 000
，如果
你有耐心的话）个文档的集合：
> for (i=0; i<1000000; i++) {
... db.users.insertOne(
... {
... "i" : i,
64
｜
第
5
章
... "username" : ...
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