The index type of MongoDB learning note 25 MongoDB

Create an index You can specify options, and indexes that are built with different options behave differently.

First, unique index

A unique index ensures that each document has a unique value for the specified key. For example, to determine that the username key in a document is not duplicated, create a unique index:

> Db.users.findOne () {"_id": ObjectId ("54ad5826245d1b7d58b53238"), "I": 2, "username": "User2", "Age": "Created": Isodate ("2015-01-07t16:00:38.634z")}

> Db.users.ensureIndex ({"username": 1},{"unique": true}) {"createdcollectionautomatically": false, "num Indexesbefore ": 1," Numindexesafter ": 2," OK ": 1}

> Db.users.insert ({"username": "User2"}) Writeresult ({"ninserted": 0, "Writeerror": {"Co De ": 11000," errmsg ":" Insertdocument:: Caused by:: 11000 E11000 duplicate key error index:blog.users. $use Rname_1 dup key: {: \ "user2\"} "}}) >

You can see that after creating a unique index, you cannot insert the same key value;

1. Unique composite Index

Create a unique composite index, the single-key value of a composite index can be duplicated, but the combined key values that make up a unique composite index can only be unique. For example:

If there is a {"username": "User2", "Age": 18} has a unique index, the following insert is legal,

>db.users.insert ({"username": "Bob"}) >db.users.insert ({"username": "Bob", "Age": All}) >db.users.insert ({" Username ":" Fred "," Age ": 23})

If you insert any of these three items again, it will cause the key to repeat the exception;

2. To repeat

Creating a unique index on a field that already has duplicate values causes an error; You can use the Dropdups option to encounter duplicate values that retain the first, removing other duplicates;

>db.users.ensureindex ({"username": 1},{"unique": true, "dropdups": true})

Use this option with caution, because there is no way to control which documents are deleted;

Second, sparse index

If there is a field that may or may not exist, the index created on this field is called a sparse index.

To create a unique sparse index:

>db.users.ensureindex ({"username": 1},{"unique": true, "sparse": true})

To create a non-unique sparse index:

>db.users.ensureindex ({"username": 1},{"sparse": true})

This article is from the "Margin with Wish" blog, please be sure to keep this source http://281816327.blog.51cto.com/907015/1601478

The index type of MongoDB learning note 25 MongoDB