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Create an Index on an Embedded Document
You can create indexes on embedded documents as a whole. However, only queries that specify the entire embedded document use the index. Queries on a specific field within the document do not use the index.
About this Task
To utilize an index on an embedded document, your query must specify the entire embedded document. This can lead to unexpected behaviors if your schema model changes and you add or remove fields from your indexed document.
When you query embedded documents, the order that you specify fields in the query matters. The embedded documents in your query and returned document must match exactly. To see examples of queries on embedded documents, see Query on Embedded/Nested Documents.
Before you create an index on an embedded document, consider if you should instead index specific fields in that document, or use a wildcard index to index all of the document's subfields.
Before you Begin
Create a students collection that contains the following documents:
db.students.insertMany( [ { "name": "Alice", "gpa": 3.6, "location": { city: "Sacramento", state: "California" } }, { "name": "Bob", "gpa": 3.2, "location": { city: "Albany", state: "New York" } } ] )
Steps
Create an index on the location field:
db.students.createIndex( { location: 1 } )
Results
The following query uses the index on the location field:
db.students.find( { location: { city: "Sacramento", state: "California" } } )
The following queries do not use the index on the location field
because they query on specific fields within the embedded document:
db.students.find( { "location.city": "Sacramento" } ) db.students.find( { "location.state": "New York" } )
In order for a dot notation query to use an index, you must create an index on the specific embedded field you are querying, not the entire embedded object. For an example, see Create an Index on an Embedded Field.
The following query returns no results because the embedded fields in the query predicate are specified in a different order than they appear in the document:
db.students.find( { location: { state: "California", city: "Sacramento" } } )