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MongoDB - Indexing

Indexes support the efficient resolution of queries. Without indexes, MongoDB must scan every document of a collection to select those documents that match the query statement. This scan is highly inefficient and requires MongoDB to process a large volume of data.

Indexes are special data structures that store a small portion of the data set in an easy-to-traverse form. The index stores the value of a specific field or set of fields, ordered by the value of the field as specified in the index.

The createIndex() Method​

To create an index, you need to use the createIndex() method of MongoDB.

Syntax​

The basic syntax of the createIndex() method is as follows:

>db.COLLECTION_NAME.createIndex({KEY:1})

Here, KEY is the name of the field on which you want to create an index, and 1 is for ascending order. To create an index in descending order, you need to use -1.

Example​

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

In the createIndex() method, you can pass multiple fields to create an index on multiple fields.

>db.mycol.createIndex({"title":1,"description":-1})

Index Options​

This method also accepts a list of options (which are optional). Following is the list:

ParameterTypeDescription
backgroundBooleanBuilds the index in the background so that building an index does not block other database activities. Default value is false.
uniqueBooleanCreates a unique index so that the collection will not accept insertion of documents where the index key matches an existing value. Specify true to create a unique index. Default value is false.
nameStringThe name of the index. If unspecified, MongoDB generates an index name by concatenating the names of the indexed fields and the sort order.
sparseBooleanIf true, the index only references documents with the specified field. These indexes use less space but behave differently in some situations (particularly sorts). Default value is false.
expireAfterSecondsIntegerSpecifies a value, in seconds, as a TTL to control how long MongoDB retains documents in this collection.
weightsDocumentThe weight is a number ranging from 1 to 99,999 and denotes the significance of the field relative to other indexed fields in terms of the score.
default_languageStringFor a text index, the language that determines the list of stop words and the rules for the stemmer and tokenizer. Default value is English.
language_overrideStringFor a text index, specify the name of the field in the document that contains the language to override the default language. Default value is language.

The dropIndex() Method​

You can drop a particular index using the dropIndex() method of MongoDB.

Syntax​

The basic syntax of the dropIndex() method is as follows:

>db.COLLECTION_NAME.dropIndex({KEY:1})

Here, KEY is the name of the field on which you want to remove an existing index. Instead of the index specification document (above syntax), you can also specify the name of the index directly as:

dropIndex("name_of_the_index")

Example​

> db.mycol.dropIndex({"title":1})
{
    "ok" : 0,
    "errmsg" : "can't find index with key: { title: 1.0 }",
    "code" : 27,
    "codeName" : "IndexNotFound"
}

The dropIndexes() Method​

This method deletes multiple (specified) indexes on a collection.

Syntax​

The basic syntax of the dropIndexes() method is as follows:

>db.COLLECTION_NAME.dropIndexes()

Example​

Assume we have created 2 indexes in the named mycol collection as shown below:

> db.mycol.createIndex({"title":1,"description":-1})

Following example removes the above-created indexes of mycol:

>db.mycol.dropIndexes({"title":1,"description":-1})
{ "nIndexesWas" : 2, "ok" : 1 }

The getIndexes() Method​

This method returns the description of all the indexes in the collection.

Syntax​

Following is the basic syntax of the getIndexes() method:

db.COLLECTION_NAME.getIndexes()

Example​

Assume we have created 2 indexes in the named mycol collection as shown below:

> db.mycol.createIndex({"title":1,"description":-1})

Following example retrieves all the indexes in the collection mycol:

> db.mycol.getIndexes()
[
    {
        "v" : 2,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "test.mycol"
    },
    {
        "v" : 2,
        "key" : {
            "title" : 1,
            "description" : -1
        },
        "name" : "title_1_description_-1",
        "ns" : "test.mycol"
    }
]

Diagram​

Note​

Indexes can greatly improve query performance, but they also come with overhead. It's important to balance the benefits of indexing with the cost of maintaining them, especially for write-heavy operations.