mongoDB : Indexing documents

“ensureIndex” method is used to creates an index on the specified field if the index does not already exist.

Syntax

db.collection_name.ensureIndex(keys, options)

Parameter	Type	Description
keys	document	Document contains key-value pairs, where the field is the index key and the value describes the type of index for that field. For an ascending index on a field, specify a value of 1; for descending index, specify a value of -1.
options	document	Optional. A document that contains a set of options that controls the creation of the index.

options

There are set of options available to control the creation of index.

Parameter	Type	Description
background	boolean	Optional. 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	Optional. 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	Optional. The name of the index. If unspecified, MongoDB generates an index name by concatenating the names of the indexed fields and the sort order.
sparse	boolean	Optional. 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	Optional. Specifies a value, in seconds, as a TTL(Time to live) to control how long MongoDB retains documents in this collection.
v	index version	Optional. The index version number. The default index version depends on the version of mongod running when creating the index.

For more information about options, refer following link.

http://docs.mongodb.org/manual/reference/method/db.collection.ensureIndex/

db.employee.ensureIndex({"firstName":1,"lastName":-1})

Above statement creates index on fields “firstName”, “lastName”. “firstName” in Ascending order, “lastName” in descending order.

> db.employee.ensureIndex({"firstName":1,"lastName":-1})
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}
>

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