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Create an Index on an Array Field

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About this Task

Procedure
Results
Learn More

You can create an index on a field containing an array value to improve performance for queries on that field. When you create an index on a field containing an array value, MongoDB stores that index as a multikey index.

To create an index, use the db.collection.createIndex() method. Your operation should resemble this prototype:

db.<collection>.createIndex( { <field>: <sortOrder> } )

About this Task

The example on this page uses a students collection that contains these documents:

db.students.insertMany( [
   {
      "name": "Andre Robinson",
      "test_scores": [ 88, 97 ]
   },
   {
      "name": "Wei Zhang",
      "test_scores": [ 62, 73 ]
   },
   {
      "name": "Jacob Meyer",
      "test_scores": [ 92, 89 ]
   }
] )

You regularly run a query that returns students with at least one test_score greater than 90. You can create an index on the test_scores field to improve performance for this query.

Procedure

The following operation creates an ascending multikey index on the test_scores field of the students collection:

db.students.createIndex( { test_scores: 1 } )

Because test_scores contains an array value, MongoDB stores this index as a multikey index.

Results

The index contains a key for each individual value that appears in the test_scores field. The index is ascending, meaning the keys are stored in this order: [ 62, 73, 88, 89, 92, 97 ].

The index supports queries that select on the test_scores field. For example, the following query returns documents where at least one element in the test_scores array is greater than 90:

db.students.find(
   {
      test_scores: { $elemMatch: { $gt: 90 } }
   }
)

Output:

[
   {
      _id: ObjectId("632240a20646eaee87a56a80"),
      name: 'Andre Robinson',
      test_scores: [ 88, 97 ]
   },
   {
      _id: ObjectId("632240a20646eaee87a56a82"),
      name: 'Jacob Meyer',
      test_scores: [ 92, 89 ]
   }
]

Learn More

To learn how to create a multikey index on embedded document fields, see Create an Index on an Embedded Field in an Array.
To learn about multikey index bounds, see Multikey Index Bounds.

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