MongoDB Indexing - Create, Find and Drop Index

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In this post, we will see how to create, find and drop indexes in MongoDB.

Indexes support the efficient execution of queries in MongoDB. Without indexes, MongoDB must perform a collection scan, i.e. scan every document in a collection, to select those documents that match the query statement. If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect.

In this post, you will learn –

• How to Create Indexes: createIndex()
• How to Find Indexes: getIndexes()
• How to Drop Indexes: dropIndex()

1. How to Create Indexes: createIndex()

Creating an Index in MongoDB is done by using the "createIndex()" method.

Syntax:

db.collection_name.createIndex({field_name: 1 or -1})

• The value 1 is for ascending order and -1 is for descending order.
• The db.collection.createIndex() method only creates an index if an index of the same specification does not already exist.
• MongoDB indexes use a B-tree data structure.

Example

Let's say we want to create the index on title field of a post in ascending order:

> db.posts.createIndex({title: 1});
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}

Complete example:

> db.posts.createIndex({title: 1});
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}
> db.posts.find().pretty();
{
        "_id" : ObjectId("5e184a067695f4d696a0598d"),
        "title" : "MongoDB Overview",
        "description" : "MongoDB is no sql database",
        "by" : "Java Guides",
        "url" : "https://javaguides.net",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 100
}
{
        "_id" : ObjectId("5e184a067695f4d696a0598e"),
        "title" : "NoSQL Database",
        "description" : "NoSQL database doesn't have tables",
        "by" : "Java Guides",
        "url" : "https://javaguides.net",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 20,
        "comments" : [
                {
                        "user" : "user1",
                        "message" : "My first comment",
                        "dateCreated" : ISODate("2013-12-10T09:35:00Z"),
                        "like" : 0
                }
        ]
}
{
        "_id" : ObjectId("5e18544a7695f4d696a0598f"),
        "title" : "MongoDB CRUD Operations",
        "description" : "MongoDB CRUD Operations",
        "by" : "Java Guides",
        "url" : "https://javaguides.net",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 20,
        "comments" : [
                {
                        "user" : "user1",
                        "message" : "My first comment",
                        "dateCreated" : ISODate("2013-12-10T09:35:00Z"),
                        "like" : 0
                }
        ]
}

How to Find Indexes: getIndexes()

Finding an Index in MongoDB is done by using the "getIndexes()" method.

Syntax

db.collection_name.getIndexes()

Example

> db.posts.getIndexes();
[
        {
                "v" : 2,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "mydb.posts"
        },
        {
                "v" : 2,
                "key" : {
                        "title" : 1
                },
                "name" : "title_1",
                "ns" : "mydb.posts"
        }
]

The output shows that we have two indexes in this collection. The default index created on _id and the index that we have created on title field.

How to Drop Indexes: dropIndex()

Removing an Index in MongoDB is done by using the "dropIndex()" method.

We can either drop a particular index or all the indexes.

Dropping a specific index

db.collection_name.dropIndex({index_name: 1})

Let's drop the index that we have created on the title field in the collection posts. The command for this:

> db.posts.dropIndex({title: 1});
{ "nIndexesWas" : 2, "ok" : 1 }

nIndexesWas: It shows how many indexes were there before this command got executed ok: 1: This means the command is executed successfully.

Dropping all the indexes:

To drop all the indexes of a collection, we use dropIndexes() method.

Syntax of dropIndexs() method:

db.collection_name.dropIndexes()

Let's drop all the indexes of posts collection.

> db.posts.dropIndexes();
{
        "nIndexesWas" : 1,
        "msg" : "non-_id indexes dropped for collection",
        "ok" : 1
}

Summary

The below diagram shows the summary of all the commands used in this post: