Multi-key Indexes

For fields that have an array value, a multi-key index allows you to create an index key for each element in the array. Indexing an array creates an index entry for each element of the array.

Multi-key indexes are beneficial when your application frequently queries or filters documents based on values in arrays.

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 multi-key index

Use the createIndex() method to create a multi-key index. The method 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>
}

The following example creates a multi-key index on the categories field sorted in ascending order with the name book_categories:

db.collection.createIndex(
  {
    "categories": 1
  },
  {
    "name": "book_categories"
  }
)

See Index Properties for examples of creating multi-key indexes.