Multikey Indexes

To index a field that holds an array value, MongoDB creates an index key for each element in the array. These multikey indexes support efficient queries against array fields. Multikey indexes can be constructed over arrays that hold both scalar values (e.g. strings, numbers) and nested documents.

Diagram of a multikey index on the ``addr.zip`` field. The ``addr`` field contains an array of address documents. The address documents contain the ``zip`` field.

Create Multikey Index

To create a multikey index, use the db.collection.createIndex() method:

db.coll.createIndex( { <field>: < 1 or -1 > } )

MongoDB automatically creates a multikey index if any indexed field is an array; you do not need to explicitly specify the multikey type.

Index Bounds

If an index is multikey, then computation of the index bounds follows special rules. For details on multikey index bounds, see Multikey Index Bounds.

Limitations

Compound Multikey Indexes

For a compound multikey index, each indexed document can have at most one indexed field whose value is an array. As such, you cannot create a compound multikey index if more than one to-be-indexed field of a document is an array. Or, if a compound multikey index already exists, you cannot insert a document that would violate this restriction.

For example, consider a collection that contains the following document:

{ _id: 1, a: [ 1, 2 ], b: [ 1, 2 ], category: "AB - both arrays" }

You cannot create a compound multikey index { a: 1, b: 1 } on the collection since both the a and b fields are arrays.

But consider a collection that contains the following documents:

{ _id: 1, a: [1, 2], b: 1, category: "A array" }
{ _id: 2, a: 1, b: [1, 2], category: "B array" }

A compound multikey index { a: 1, b: 1 } is permissible since for each document, only one field indexed by the compound multikey index is an array; i.e. no document contains array values for both a and b fields. After creating the compound multikey index, if you attempt to insert a document where both a and b fields are arrays, MongoDB will fail the insert.

Shard Keys

You cannot specify a multikey index as the shard key index.

Changed in version 2.6: However, if the shard key index is a prefix of a compound index, the compound index is allowed to become a compound multikey index if one of the other keys (i.e. keys that are not part of the shard key) indexes an array. Compound multikey indexes can have an impact on performance.

Hashed Indexes

Hashed indexes cannot be multikey.

Covered Queries

A multikey index cannot support a covered query.

Query on the Array Field as a Whole

When a query filter specifies an exact match for an array as a whole, MongoDB can use the multikey index to look up the first element of the query array but cannot use the multikey index scan to find the whole array. Instead, after using the multikey index to look up the first element of the query array, MongoDB retrieves the associated documents and filters for documents whose array matches the array in the query.

For example, consider an inventory collection that contains the following documents:

{ _id: 5, type: "food", item: "aaa", ratings: [ 5, 8, 9 ] }
{ _id: 6, type: "food", item: "bbb", ratings: [ 5, 9 ] }
{ _id: 7, type: "food", item: "ccc", ratings: [ 9, 5, 8 ] }
{ _id: 8, type: "food", item: "ddd", ratings: [ 9, 5 ] }
{ _id: 9, type: "food", item: "eee", ratings: [ 5, 9, 5 ] }

The collection has a multikey index on the ratings field:

db.inventory.createIndex( { ratings: 1 } )

The following query looks for documents where the ratings field is the array [ 5, 9 ]:

db.inventory.find( { ratings: [ 5, 9 ] } )

MongoDB can use the multikey index to find documents that have 5 at any position in the ratings array. Then, MongoDB retrieves these documents and filters for documents whose ratings array equals the query array [ 5, 9 ].

Examples

Index Basic Arrays

Consider a survey collection with the following document:

{ _id: 1, item: "ABC", ratings: [ 2, 5, 9 ] }

Create an index on the field ratings:

db.survey.createIndex( { ratings: 1 } )

Since the ratings field contains an array, the index on ratings is multikey. The multikey index contains the following three index keys, each pointing to the same document:

  • 2,
  • 5, and
  • 9.

Index Arrays with Embedded Documents

You can create multikey indexes on array fields that contain nested objects.

Consider an inventory collection with documents of the following form:

{
  _id: 1,
  item: "abc",
  stock: [
    { size: "S", color: "red", quantity: 25 },
    { size: "S", color: "blue", quantity: 10 },
    { size: "M", color: "blue", quantity: 50 }
  ]
}
{
  _id: 2,
  item: "def",
  stock: [
    { size: "S", color: "blue", quantity: 20 },
    { size: "M", color: "blue", quantity: 5 },
    { size: "M", color: "black", quantity: 10 },
    { size: "L", color: "red", quantity: 2 }
  ]
}
{
  _id: 3,
  item: "ijk",
  stock: [
    { size: "M", color: "blue", quantity: 15 },
    { size: "L", color: "blue", quantity: 100 },
    { size: "L", color: "red", quantity: 25 }
  ]
}

...

The following operation creates a multikey index on the stock.size and stock.quantity fields:

db.inventory.createIndex( { "stock.size": 1, "stock.quantity": 1 } )

The compound multikey index can support queries with predicates that include both indexed fields as well as predicates that include only the index prefix "stock.size", as in the following examples:

db.inventory.find( { "stock.size": "M" } )
db.inventory.find( { "stock.size": "S", "stock.quantity": { $gt: 20 } } )

For details on how MongoDB can combine multikey index bounds, see Multikey Index Bounds. For more information on behavior of compound indexes and prefixes, see compound indexes and prefixes.

The compound multikey index can also support sort operations, such as the following examples:

db.inventory.find( ).sort( { "stock.size": 1, "stock.quantity": 1 } )
db.inventory.find( { "stock.size": "M" } ).sort( { "stock.quantity": 1 } )

For more information on behavior of compound indexes and sort operations, see Use Indexes to Sort Query Results.