MongoDB Multikey Indexes

Indexes are special data structures that store some information related to the documents such that it becomes easy for MongoDB to find the right data file. They also store the value of a specific field or set of fields, ordered by the value of the field as specified in the index.

MongoDB allows indexing a field that holds an array value by creating an index key for each element in the array, such type of indexing is called Multikey Indexes.

Multikey Indexes in MongoDB

MongoDB multikey indexes are used to collect and sort data from fields containing array values. MongoDB automatically creates a multikey index when a field that contains an array value is indexed.

Multikey indexes support efficient queries against array fields. It can be constructed over arrays that hold both scalar values(like strings, numbers, etc) and nested documents

How to Create a Multikey Index?

To create a Multikey Index in MongoDB use the createIndex() method.

Syntax

The syntax to create a Multikey Index is given below:

db.Collection_name.createIndex({filed_name: 1/ -1})

MongoDB Multikey Indexes Examples

In the following example, we are working with:

Database: gfg

Collections: student

Document: Three documents contains the details of the students

Example 1: Creating a multikey index

Now we create multi-index with the help of createIndex() method on the field language:

Query:

db.student.createIndex({language:1})

After indexing, we will check the index using the getIndexes() method:

Example 2: Indexing an array with the embedded document

You are allowed to create a multikey index on an array field that contains nested documents/objects.

db.student.createIndex({"details.age":1, "details.branch":1})

Here, we create the multikey index on the “details.age” and “details.branch” fields.

After indexing, we will check the index using the getIndexes() method:

Limitations:

• When you are creating a compound multikey index then each indexed document must contain at most one indexed field whose value is an array.
• You are not allowed to create a compound multikey index if more than one to be indexed field of a document is an array.
• When the compound multikey index already exists, then you are not allowed to insert a document that will break the restrictions.
• When a field is an array of documents, you are allowed to index an embedded document to create a compound index but at most one indexed field can be an array.

KeyTakeAways About MongoDB Multikey Indexes

• Multikey indexes in MongoDB are used to collect and sort data from fields containing array values, improving query performance for array fields.
• You are not allowed to specify a multikey index as the sherd key index.
• In MongoDB, hashed indexes are not multikey index.
• The multikey index cannot support the $expr operator.
• In MongoDB, if a filter query specifies the exact match for an array as a whole, then MongoDB scans the multikey index to look for the first element of the quarry array.