Database Manual / Indexes / Properties

Unique Indexes

A unique index ensures that the indexed fields do not store duplicate values. A unique index on a single field ensures that a value appears at most once for a given field. A unique compound index ensures that any given combination of the index key values only appears at most once. By default, MongoDB creates a unique index on the _id field during the creation of a collection.

You can create and manage unique indexes in the UI for deployments hosted in MongoDB Atlas.

Create a Unique Index

To create a unique index, use the db.collection.createIndex() method with the unique option set to true.

db.collection.createIndex( <key and index type specification>, { unique: true } )

Unique Index on a Single Field

For example, to create a unique index on the user_id field of the members collection, use the following operation in mongosh:

db.members.createIndex( { "user_id": 1 }, { unique: true } )

Unique Compound Index

You can also enforce a unique constraint on compound indexes. A unique compound index enforces uniqueness on the combination of the index key values.

For example, to create a unique index on groupNumber, lastname, and firstname fields of the members collection, use the following operation in mongosh:

db.members.createIndex( { groupNumber: 1, lastname: 1, firstname: 1 }, { unique: true } )

The created index enforces uniqueness for the combination of groupNumber, lastname, and firstname values.

For another example, consider a collection with the following document:

{ _id: 1, a: [ { loc: "A", qty: 5 }, { qty: 10 } ] }

Create a unique compound multikey index on a.loc and a.qty:

db.collection.createIndex( { "a.loc": 1, "a.qty": 1 }, { unique: true } )

The unique index permits the insertion of the following documents into the collection since the index enforces uniqueness for the combination of a.loc and a.qty values:

db.collection.insertMany( [
   { _id: 2, a: [ { loc: "A" }, { qty: 5 } ] },
   { _id: 3, a: [ { loc: "A", qty: 10 } ] }
] )

Tip

Behavior

Restrictions

MongoDB cannot create a unique index on the specified index field(s) if the collection already contains data that would violate the unique constraint for the index.

You may not specify a unique constraint on a hashed index.

Building Unique Index on Replica Sets and Sharded Clusters

For replica sets and sharded clusters, using a rolling procedure to create a unique index requires that you stop all writes to the collection during the procedure. If you cannot stop all writes to the collection during the procedure, do not use the rolling procedure. Instead, to build your unique index on the collection you must either:

  • Run db.collection.createIndex() on the primary for a replica set

  • Run db.collection.createIndex() on the mongos for a sharded cluster

Unique Constraint Across Separate Documents

The unique constraint applies to separate documents in the collection. That is, the unique index prevents separate documents from having the same value for the indexed key.

Because the 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. In this case, the repeated index entry is inserted into the index only once.

For example, consider a collection with the following documents:

{ _id: 1, a: [ { loc: "A", qty: 5 }, { qty: 10 } ] }
{ _id: 2, a: [ { loc: "A" }, { qty: 5 } ] }
{ _id: 3, a: [ { loc: "A", qty: 10 } ] }

Create a unique compound multikey index on a.loc and a.qty:

db.collection.createIndex( { "a.loc": 1, "a.qty": 1 }, { unique: true } )

The unique index permits the insertion of the following document into the collection if no other document in the collection has an index key value of { "a.loc": "B", "a.qty": null }.

db.collection.insertOne( { _id: 4, a: [ { loc: "B" }, { loc: "B" } ] } )

Missing Document Field in a Unique Single-Field Index

If a document has a null or missing value for the indexed field in a unique single-field index, the index stores a null value for that document. Because of the unique constraint, a single-field unique index can only contain one document that contains a null value in its index entry. If there is more than one document with a null value in its index entry, the index build fails with a duplicate key error.

For example, a collection has a unique single-field index on x:

db.collection.createIndex( { "x": 1 }, { unique: true } )

The unique index allows the insertion of a document without the field x if the collection does not already contain a document missing the field x:

db.collection.insertOne( { y: 1 } )

However, you cannot insert a document without the field x if the collection already contains a document missing the field x:

db.collection.insertOne( { z: 1 } )

The operation fails to insert the document because of the violation of the unique constraint on the value of the field x:

WriteResult({
   "nInserted" : 0,
   "writeError" : {
      "code" : 11000,
      "errmsg" : "E11000 duplicate key error index: test.collection.$a.b_1 dup key: { : null }"
   }
})

Missing Document Fields in a Unique Compound Index

If a document has a null or missing value for one or more indexed fields in a unique compound index, the index stores a null value for each null or missing field in the document's index entry. Because of the unique constraint, a unique compound index only permits one document that has a null value for all indexed fields in an index entry. If there is more than one index entry with a null value for all indexed fields, the index build fails with a duplicate key error. MongoDB permits multiple documents with missing fields in unique compound indexes as long as each index entry is unique.

For example, a collection students has a unique compound index on fields name, age, and grade:

db.students.createIndex(
   {
      "name": 1,
      "age": -1,
      "grade": 1
   },
   { unique: true }
)

If the collection does not already contain identical documents, the unique compound index allows the insertion of the following documents that are all missing the grade field.

db.students.insertMany( [
      { "name": "Meredith", "age": 12 },
      { "name": "Olivia", "age": 11 },
      { "name": "Benjamin" }
] )

However, you cannot insert a document that has the same index key (value for name, age, and grade) as another document in the collection.

db.students.insertOne( { name: "Meredith", age: 12 } )

The operation fails to insert the document because of the violation of the unique constraint on the values of the fields name, age, and grade:

WriteResult({
   "nInserted" : 0,
   "writeError" : {
      "code" : 11000,
      "errmsg" :
         "E11000 duplicate key error collection: test.students
         index: name_1_age_-1_grade_1
         dup key: { name: "Meredith", age: 12, grade: null }
   }
} )

You also cannot insert a document that is unique but shares an index key with an existing index entry.

db.students.insertOne( { name: "Olivia", "age": 11, "favorite color": "red"} )

The operation fails to insert the document because of the violation of the unique constraint on the values of the fields name, age, and grade:

WriteResult({
   "nInserted" : 0,
   "writeError" : {
      "code" : 11000,
      "errmsg" :
         "E11000 duplicate key error collection: test.students
         index: name_1_age_-1_grade_1
         dup key: { name: "Olivia", age: 11, grade: null }
   }
} )

Unique Partial Indexes

Partial indexes only index the documents in a collection that meet a specified filter expression. If you specify both the partialFilterExpression and a unique constraint, the unique constraint only applies to the documents that meet the filter expression.

A partial index with a unique constraint does not prevent the insertion of documents that do not meet the unique constraint if the documents do not meet the filter criteria. For an example, see Partial Index with Unique Constraint.

Sharded Clusters and Unique Indexes

You cannot specify a unique constraint on a hashed index.

For a ranged sharded collection, only the following indexes can be unique:

  • the index on the shard key
  • a compound index where the shard key is a prefix

  • The default _id index; however, the _id index only enforces the uniqueness constraint per shard if the _id field is not the shard key.

Important

Sharded clusters do not enforce the uniqueness constraint on _id fields across the cluster when the _id field is not the shard key.

If the _id field is the not the shard key the uniqueness constraint only applies to the shard that stores the document. This means that two or more documents can have the same _id value, provided they occur on different shards.

For example, consider a sharded collection with shard key {x: 1} that spans two shards A and B. Because the _id key is not the shard key, the collection could have a document with _id value 1 in shard A and another document with _id value 1 in shard B.

In cases where the _id field is not the shard key, MongoDB expects applications to enforce the uniqueness of _id values across the shards.

The unique index constraints mean that:

  • For a to-be-sharded collection, you cannot shard the collection if the collection has multiple unique indexes unless the shard key is the prefix for all the unique indexes.

  • For an already-sharded collection, you cannot create unique indexes on other fields unless the shard key is included as the prefix.

  • A unique index stores a null value for a document missing the indexed field; that is a missing index field is treated as another instance of a null index key value. For more information, see Missing Document Field in a Unique Single-Field Index.

To maintain uniqueness on a field that is not your shard key, see Unique Constraints on Arbitrary Fields.

Sparse and Non-Sparse Unique Indexes

Starting in MongoDB 5.0, unique sparse and unique non-sparse indexes with the same key pattern can exist on a single collection.

Unique and Sparse Index Creation

This example creates multiple indexes with the same key pattern and different sparse options:

db.scoreHistory.createIndex( { score : 1 }, { name: "unique_index", unique: true } )
db.scoreHistory.createIndex( { score : 1 }, { name: "unique_sparse_index", unique: true, sparse: true } )

Basic and Sparse Index Creation

You can also create basic indexes with the same key pattern with and without the sparse option:

db.scoreHistory.createIndex( { score : 1 }, { name: "sparse_index", sparse: true } )
db.scoreHistory.createIndex( { score : 1 }, { name: "basic_index" } )

Basic and Unique Indexes With Duplicate Key Patterns

Starting in MongoDB 5.0, basic and unique indexes can exist with the same key pattern.

This duplication in key patterns allows for adding a unique index to already indexed fields.

In this example:

Create a basic index with the key pattern { score : 1 } and insert three documents.

db.scoreHistory.createIndex( { score : 1 }, { name: "basic_index" } )
db.scoreHistory.insert( { score : 1 } )
db.scoreHistory.insert( { score : 2 } )
db.scoreHistory.insert( { score : 3 } )

Create a unique index with the same key pattern { score : 1 }.

db.scoreHistory.createIndex( { score : 1 }, { name: "unique_index", unique: true } )

Try to insert a duplicate score document that fails because of the unique index.

db.scoreHistory.insert( { score : 3 } )
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