Single Field Indexes
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MongoDB provides complete support for indexes on any field in a collection of documents. By default, all collections have an index on the _id field, and applications and users may add additional indexes to support important queries and operations.
This document describes ascending/descending indexes on a single field.
Create an Ascending Index on a Single Field
Consider a collection named records
that holds documents that
resemble the following sample document:
{ "_id": ObjectId("570c04a4ad233577f97dc459"), "score": 1034, "location": { state: "NY", city: "New York" } }
The following operation creates an ascending index on the score
field of the records
collection:
db.records.createIndex( { score: 1 } )
The value of the field in the index specification describes the kind of
index for that field. For example, a value of 1
specifies an index
that orders items in ascending order. A value of -1
specifies an
index that orders items in descending order. For additional index
types, see index types.
The created index will support queries that select on the field
score
, such as the following:
db.records.find( { score: 2 } ) db.records.find( { score: { $gt: 10 } } )
Create an Index on an Embedded Field
You can create indexes on fields within embedded documents, just as you can index top-level fields in documents. Indexes on embedded fields differ from indexes on embedded documents, which include the full content up to the maximum index size of the embedded document in the index. Instead, indexes on embedded fields allow you to use a "dot notation," to introspect into embedded documents.
Consider a collection named records
that holds documents that
resemble the following sample document:
{ "_id": ObjectId("570c04a4ad233577f97dc459"), "score": 1034, "location": { state: "NY", city: "New York" } }
The following operation creates an index on the location.state
field:
db.records.createIndex( { "location.state": 1 } )
The created index will support queries that select on the field
location.state
, such as the following:
db.records.find( { "location.state": "CA" } ) db.records.find( { "location.city": "Albany", "location.state": "NY" } )
Create an Index on Embedded Document
You can also create indexes on embedded document as a whole.
Consider a collection named records
that holds documents that
resemble the following sample document:
{ "_id": ObjectId("570c04a4ad233577f97dc459"), "score": 1034, "location": { state: "NY", city: "New York" } }
The location
field is an embedded document, containing the embedded fields
city
and state
. The following command creates an index on the location
field as a whole:
db.records.createIndex( { location: 1 } )
The following query can use the index on the location
field:
db.records.find( { location: { city: "New York", state: "NY" } } )
Note
Although the query can use the index, the result set does not include the sample document above. When performing equality matches on embedded documents, field order matters and the embedded documents must match exactly. See Query Embedded Documents for more information regarding querying on embedded documents.
Additional Considerations
Applications may encounter reduced performance during index builds, including limited read/write access to the collection. For more information on the index build process, see Index Builds on Populated Collections, including the Index Builds in Replicated Environments section.
Some drivers may specify indexes, using NumberLong(1)
rather than
1
as the specification. This does not have any affect on the
resulting index.