Multikey Indexes

Compound Multikey Indexes

For a compound multikey index, each indexed document can have at most one indexed field whose value is an array. That is:

• You cannot create a compound multikey index if more than one to-be-indexed field of a document is an array. 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.

• Or, if a compound multikey index already exists, you cannot insert a document that would violate this restriction.

  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.

  However, 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.

If a field is an array of documents, you can index the embedded fields to create a compound index. For example, consider a collection that contains the following documents:

{ _id: 1, a: [ { x: 5, z: [ 1, 2 ] }, { z: [ 1, 2 ] } ] }
{ _id: 2, a: [ { x: 5 }, { z: 4 } ] }

You can create a compound index on { "a.x": 1, "a.z": 1 }. The restriction where at most one indexed field can be an array also applies.

For an example, see Index Arrays with Embedded Documents.

Sorting

As a result of changes to sorting behavior on array fields in MongoDB 4.4, when you sort on an array indexed with a multikey index, the query plan includes a blocking sort stage, unless:

• The index boundaries for all sort fields are [MinKey, MaxKey], and

• No boundaries for any multikey-indexed field have the same path prefix as the sort pattern.

Shard Keys

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

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

Multikey indexes cannot cover queries over array field(s).

Multikey indexes can cover queries over non-array fields if the index tracks which field or fields cause the index to be multikey.

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 ].

$expr

$expr does not support multikey indexes.

Index Basic Arrays

Create a survey collection with the following document:

db.survey.insertOne(
   { _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.