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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 [1] (e.g. strings, numbers) and nested documents.

[1]	A scalar value refers to value that is neither an embedded document nor an array.

Create Multikey Index

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

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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.

Changed in version 3.4: For the WiredTiger and In-Memory storage engines only,

Starting in MongoDB 3.4, for multikey indexes created using MongoDB 3.4 or later, MongoDB keeps track of which indexed field or fields cause an index to be a multikey index. Tracking this information allows the MongoDB query engine to use tighter index bounds.

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.

Unique Multikey Index

For unique indexes, the unique constraint applies across separate documents in the collection rather than within a single document.

Because the unique constraint applies to separate documents, for a unique multikey index, a document may have array elements that result in repeating index key values as long as the index key values for that document do not duplicate those of another document.

For more information, see Unique Constraint Across Separate Documents.

Limitations

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:

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  { _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:

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  { _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:

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{ _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.

See also

• Unique Constraint Across Separate Documents
• Unique Index on a Single Field

Sorting

As a result of changes to sorting behavior on array fields in MongoDB 3.6, when sorting on an array indexed with a multikey index the query plan includes a blocking SORT stage. The new sorting behavior may negatively impact performance.

In a blocking SORT, all input must be consumed by the sort step before it can produce output. In a non-blocking, or indexed sort, the sort step scans the index to produce results in the requested order.

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).

However, starting in 3.6, multikey indexes can cover queries over the non-array fields if the index tracks which field or fields cause the index to be multikey. Multikey indexes created in MongoDB 3.4 or later on storage engines other than MMAPv1 [2] track this data.

[2]	Starting in version 4.2, MongoDB removes the deprecated MMAPv1 storage engine.

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:

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{ _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:

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

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

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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.

Examples

Index Basic Arrays

Consider a survey collection with the following document:

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

Create an index on the field ratings:

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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:

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{
  _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:

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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:

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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:

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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.

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