mongodb 会自动在嵌入文档的 _id 字段上创建索引吗？

Question

在 node.js 项目上使用 mongodb 和 mongoose，我想知道我是否必须在嵌入项目的 _id 字段上确保索引？

也就是说，我看到 mongodb 自动在集合的 _id 字段上创建索引，它是否也为嵌入集合的 _id 字段做同样的事情？

{
   _id: "joe",
   name: "Joe Bookreader",
   addresses: [
                {
                  _id: "someid1",
                  street: "123 Fake Street",
                  city: "Faketon",
                  state: "MA",
                  zip: "12345"
                },
                {
                  _id: "someid2",
                  street: "1 Some Other Street",
                  city: "Boston",
                  state: "MA",
                  zip: "12345"
                }
              ]
 }

Answer 1

我的直觉是没有，所以我只是尝试了一下：

> db.foo.find()

> db.foo.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "foo.foo"
    }
]

> db.foo.insert({
...    _id: "joe",
...    name: "Joe Bookreader",
...    addresses: [
...                 {
...                   _id: "someid1",
...                   street: "123 Fake Street",
...                   city: "Faketon",
...                   state: "MA",
...                   zip: "12345"
...                 },
...                 {
...                   _id: "someid2",
...                   street: "1 Some Other Street",
...                   city: "Boston",
...                   state: "MA",
...                   zip: "12345"
...                 }
...               ]
...  })
WriteResult({ "nInserted" : 1 })

> db.foo.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "foo.foo"
    }
]
>

看起来没有，它不会自动在子文档上创建索引。

Answer 2

来自 MongoDB 文档：

By default, MongoDB creates a unique index on the _id field during the creation of a collection.

Answer 3

是的，根据文档 mongo dB 会在 _id 字段上创建一个唯一索引。 Default _id Index