Single Field Indexes

Single field indexes store information from any single field in a collection of documents. By default, all collections have an index on the _id field. You can add additional indexes to speed up important queries and operations.

Single field indexes are beneficial when:

• Your application frequently queries or filters documents based on the values of a specific field.

• You need to efficiently sort documents by a particular field.

• You want to ensure uniqueness for a specific field across documents in a collection by creating a unique index.

Supported index properties

Option	3.6	4.0	5.0	8.0	Elastic Cluster
name	Yes	Yes	Yes	Yes	Yes
unique	Yes	Yes	Yes	Yes	Yes
sparse *	Yes	Yes	Yes	Yes	Yes
partialFilterExpression *	No	No	Yes	Yes	No
expireAfterSeconds	Yes	Yes	Yes	Yes	Yes

* The sparse and partialFilterExpression options cannot be used together in the same index definition. If you attempt to create an index with these options, it will fail with the following error:

Error in specification: cannot mix partialFilterExpression and sparse options

Creating a single field index

Use the createIndex() method to create a single field index. The syntax is: db.collection.createIndex(<key>, <options>).

The key parameter is a JSON document that specifies the field and the index sort order:

{
  "<field>": <1 (ascending)|-1 (descending)>
}

The options parameter is a JSON document that specifies the options for the index:

{
  "name": "<name>",
  "unique": <true | false>,
  "sparse": <true | false>,
  "partialFilterExpression": <filter expression>,
  "expireAfterSeconds": <seconds before expiry>
}

See Index Properties for examples of creating single field indexes.