mongo中的稀疏索引和空值 [英] sparse indexes and null values in mongo

问题描述

我不确定我是否正确理解稀疏索引。

I'm not sure I understand sparse indexes correctly.

我在fbId上有一个稀疏的唯一索引

I have a sparse unique index on fbId

{
    "ns" : "mydb.users",
    "key" : {
        "fbId" : 1
    },
    "name" : "fbId_1",
    "unique" : true,
    "sparse" : true,
    "background" : false,
    "v" : 0
}

我希望这会允许我插入带有null的记录作为fbId，但是会引发重复的键异常。它只允许我在完全删除fbId属性时插入。

And I was expecting that would allow me to insert records with null as the fbId, but that throws a duplicate key exception. It only allows me to insert if the fbId property is removed completely.

是不是应该处理的稀疏索引？

Isn't a sparse index supposed to deal with that?

推荐答案

稀疏索引不包含错过索引字段的文档。但是，如果字段存在且值为 null ，则仍会对其进行索引。因此，如果该字段的absense及其与 null 的相等性对于您的应用程序看起来相同，并且您希望保持 fbId ，只要你有一个值就不要插入它。

Sparse indexes do not contain documents that miss indexed field. However, if field exists and has value of null, it will still be indexed. So, if absense of the field and its equality to null look the same for your application and you want to maintain uniqueness of fbId, just don't insert it until you have a value for it.

当你有大量的文件时，你需要稀疏的索引，但只有一小部分它们包含一些字段，您希望能够通过该字段快速查找文档。创建一个普通的索引太贵了，你只会浪费宝贵的RAM来索引你不感兴趣的文档。

You need sparse indexes when you have a large number of documents, but only a small portion of them contains some field, and you want to be able to quickly find documents by that field. Creating a normal index would be too expensive, you would just waste precious RAM on indexing documents you're not interested in.

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