Indexing in MongoDB

As we know index is the technique to arrange data in the database which supports the efficient resolution of queries. If we run a query to select data, MongoDB does scan every document of collection. Scanning of all document may affect the performance of MongoDB when it processes the large volume of data.

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 ensureIndex() Method

In MongoDB to create an index we need to use ensureIndex() method.

Syntax:

 Basic syntax of ensureIndex() method is given below:

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

In MongoDB, if we want to create index in ascending order, we will use 1 or descending order to use -1. 

Example

>db.testCol.ensureIndex({"title":1})

In ensureIndex() method we can pass multiple fields, to create an index on multiple fields.

>db.testCol.ensureIndex({"title":1,"description":-1})

ensureIndex() method also accepts a list of options (which are optional), whose list is given below:

Parameter	Type	Description
Background	Boolean	Builds the index in the background so that building an index does not block other database activities. Specify true to build in the background. The default value is false.
Unique	Boolean	It creates a unique index so that the collection will not accept the insertion of documents where the index key or keys match an existing value in the index. Specify true to create a unique index. The default value is false.
Name	String	The name of the index. If unspecified, MongoDB generates an index name by concatenating the names of the indexed fields and the sort order.
dropDups	Boolean	It creates a unique index on a field that may have duplicates. MongoDB indexes only the first occurrence of a key and removes all documents from the collection that contain subsequent occurrences of that key. Specify true to create a unique index. The default value is false.
Sparse	Boolean	If true, the index only references documents with the specified field. These indexes use less space but behave differently in some situations (particularly sorts). The default value is false.
expireAfterSeconds	Integer	Specifies a value, in seconds, as a TTL to control how long MongoDB retains documents in this collection.
V	index version	The index version number. The default index version depends on the version of mongod running when creating the index.
Weights	Document	The weight is a number ranging from 1 to 99,999 and denotes the significance of the field relative to the other indexed fields in terms of the score.
default_language	String	For a text index, the language that determines the list of stop words and the rules for the stemmer and tokenizer. The default value is English.
language_override	String	For a text index, specify the name of the field in the document that contains, the language to override the default language.