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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索引
｜
99
多键索引可能会比非多键索引慢一些。可能会有许多索引项指向同一个文档，因此
MongoDB
在返回结果之前可能需要做一些删除重复数据的操作。
5.1.7
索引基数
基数
（
cardinality
）
是指集合中某个字段有多少个不同的值。有些字段，比如
"gender"
或
"newsletter opt-out"
，可能只有两个值，这类键的基数就非常低。其他一些字段，比如
"username"
或
"email"
，可能集合中的每个文档都有一个不同的值，这类键的基数就非常
高。还有一些字段介于两者之间，比如
"age"
或
"zip code"
。
通常来说，一个字段的基数越高，这个字段上的索引就越有用。这是因为这样的索引能够
迅速将搜索范围缩小到一个比较小的结果集。对于基数比较低的字段，索引通常无法排除
大量可能的匹配项。
假设我们在
"gender"
上有一个索引，需要查找名字是
Susan
的女性。通过这个索引，只能
将搜索空间缩小大约
50%
，然后再引用每个单独的文档查询
"name"
字段。相反，如果在
"name"
上创建索引，就可以立即将结果集缩小到名字为
Susan
的一小部分用户，然后引用
这些文档检查性别。
根据经验来说，应该在基数比较高的键上创建索引，或者至少应该把基数比较高的键放在
复合索引的前面（在低基数的键之前）。
5.2
explain
输出
如你所见，
explain
可以为查询提供大量的信息。对于慢查询来说，它是最重要的诊断工
具之一。通过查看一个查询的
explain
输出，可以了解查询都使用了哪些索引以及是如何
使用的。对于任何查询 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.