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Indexing in MongoDB

As we know index is technique to arrange data in database which support the efficient resolution of queries. If we run a query to select data, MongoDB do scan every document of collection. Scanning of all document may affect performance of MongoDB when it process 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 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 the 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 index on multiple fields.

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


ensureIndex() method also accepts 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
Creates a unique index so that the collection will not accept 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
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 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.

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