Changed in version 3.2: Starting in MongoDB 3.2, MongoDB provides the option to create partial indexes. Partial indexes offer a superset of the functionality of sparse indexes. If you are using MongoDB 3.2 or later, partial indexes should be preferred over sparse indexes.

Note

Do not confuse sparse indexes in MongoDB with block-level indexes in other databases. Think of them as dense indexes with a specific filter.

sparse Index and Incomplete Results

If a sparse index would result in an incomplete result set for queries and sort operations, MongoDB will not use that index unless a hint() explicitly specifies the index.

For example, the query { x: { $exists: false } } will not use a sparse index on the x field unless explicitly hinted. See Sparse Index On A Collection Cannot Return Complete Results for an example that details the behavior.

If you include a hint() that specifies a sparse index when you perform a count() of all documents in a collection (i.e. with an empty query predicate), the sparse index is used even if the sparse index results in an incorrect count.

db.collection.insert({ _id: 1, y: 1 } );
db.collection.createIndex( { x: 1 }, { sparse: true } );

db.collection.find().hint( { x: 1 } ).count();

To obtain the correct count, do not hint() with a sparse index when performing a count of all documents in a collection.

db.collection.find().count();

db.collection.createIndex({ y: 1 });
db.collection.find().hint({ y: 1 }).count();

Indexes that are sparse by Default

2dsphere (version 2), 2d, geoHaystack, and text indexes are always sparse.

sparse Compound Indexes

Sparse compound indexes that only contain ascending/descending index keys will index a document as long as the document contains at least one of the keys.

For sparse compound indexes that contain a geospatial key (i.e. 2dsphere, 2d, or geoHaystack index keys) along with ascending/descending index key(s), only the existence of the geospatial field(s) in a document determine whether the index references the document.

For sparse compound indexes that contain text index keys along with ascending/descending index keys, only the existence of the text index field(s) determine whether the index references a document.

sparse and unique Properties

An index that is both sparse and unique prevents collection from having documents with duplicate values for a field but allows multiple documents that omit the key.

Create a Sparse Index On A Collection

Consider a collection scores that contains the following documents:

{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }

The collection has a sparse index on the field score:

db.scores.createIndex( { score: 1 } , { sparse: true } )

Then, the following query on the scores collection uses the sparse index to return the documents that have the score field less than ($lt) 90:

db.scores.find( { score: { $lt: 90 } } )

Because the document for the userid "newbie" does not contain the score field and thus does not meet the query criteria, the query can use the sparse index to return the results:

{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }

Sparse Index On A Collection Cannot Return Complete Results

Consider a collection scores that contains the following documents:

{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }

The collection has a sparse index on the field score:

db.scores.createIndex( { score: 1 } , { sparse: true } )

Because the document for the userid "newbie" does not contain the score field, the sparse index does not contain an entry for that document.

Consider the following query to return all documents in the scores collection, sorted by the score field:

db.scores.find().sort( { score: -1 } )

Even though the sort is by the indexed field, MongoDB will not select the sparse index to fulfill the query in order to return complete results:

{ "_id" : ObjectId("523b6e6ffb408eea0eec2649"), "userid" : "nina", "score" : 90 }
{ "_id" : ObjectId("523b6e61fb408eea0eec2648"), "userid" : "abby", "score" : 82 }
{ "_id" : ObjectId("523b6e32fb408eea0eec2647"), "userid" : "newbie" }

To use the sparse index, explicitly specify the index with hint():

db.scores.find().sort( { score: -1 } ).hint( { score: 1 } )

The use of the index results in the return of only those documents with the score field: