What is Indexing in MongoDB: A Comprehensive Guide

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What is Indexing in MongoDB?

Indexing is an integral component of achieving optimum query speed in MongoDB. It involves creating a data structure that stores document references and index keys, resulting in efficient data retrieval  

Indexes applied to MongoDB offer additional benefits as they can support string-oriented and array-based data types with greater flexibility as compared with relational database management systems.  

Benefits of Indexing in MongoDB

MongoDB indexes play a vital role in improving query performance and overall database efficiency.

Here are some key benefits of using indexes in MongoDB:

Enhanced query speed

MongoDB uses indexes to promptly find and fetch data according to specific query requirements. Indexing reduces the number of documents that require scanning, thereby increasing query speed.  

Sorting and aggregating effectively

Indexes are beneficial for boosting the efficiency of MongoDB sorting and aggregation operations. In cases where a field is utilized for grouping or sorting, creating an index enables MongoDB to perform searching and sorting operations with greater effectiveness.

Disk I/O is decreased

By making use of indexes, MongoDB can decrease the volume of data that needs to be retrieved from disk. This enhancement leads to a reduction in disk I/O operations and thus accelerates query processing times while improving system performance overall.

Index Creation and Management in MongoDB

The process to create an index and handle it in MongoDB is quite simple. Here are the steps involved:

Creating an Index  

To create indexed fields in MongoDB, utilize the createIndex() function. Indicate the name of the MongoDB collection, identify which field(s) should be indexed, and include any supplementary options relating to index configuration. In MongoDB, you can create indexes of various types.


The example above illustrates the creation of an index on field1 in ascending order and field2 in descending order.

Listing existing Indexes

If you want to see the current indexes present in a MongoDB collection, simply make use of either the getIndexes() method or execute db.collection.getIndexes() command from within the MongoDB shell.


Removing an index  

If you want to eliminate an index from a collection, the dropIndex() function comes in handy. Just indicate either the name of the collection or its respective index or specification, then proceed with deleting it.


Index management

To effectively manage different index types, MongoDB offers a range of commands and techniques. These include the ability to rebuild indexes, analyze their usage patterns, modify their properties, and more.  

To gain detailed guidance on how to manage indexes, refer to the official MongoDB documentation.

Types of MongoDB indexes

MongoDB provides different index types to suit diverse data demands and query behavior. This article will delve into the most prevalent index types found in MongoDB.

Single Field Index  

Single-field indexes are the most basic type of indexes. They're generated on a singular field and facilitate speedy querying and sorting mechanisms based solely on that particular attribute.


Here collection_name is the name of your collection.

The field is the field name on which the index is created.  

In sort order 1 or -1 in the key specification depicts ascending and descending order respectively.

Compound index

Multiple fields are used to create a compound index in MongoDB which can optimize queries by merging the fields into a single index.


Here collection_name is the name of your collection.

The field is the indexed field name.  

In sort order 1 or -1 in the indexed key specification depicts ascending and descending order respectively.

Multikey index

In MongoDB, multikey indexes are utilized for array-containing fields. Each element within the array is assigned an index key by default, allowing efficient sorting and querying of these elements.


Here collection_name is the name of your collection.

The field is the indexed field name.  

In sort order 1 or -1 in the indexed keys specification depicts ascending and descending order respectively.

Text Index

The purpose of text indexes is to facilitate full-text search functionality on string fields. With the help of these indexes, searching for specific textual patterns becomes more efficient, and language-specific rules ensure accurate word parsing.


Here collection_name is the name of your collection.

The field is the indexed field name.  

In sort order 1 or -1 in the index key specification depicts ascending and descending order respectively.

Geospatial Index

Geospatial data, like map coordinates, can be stored and retrieved using geospatial indexes. MongoDB offers two kinds of such indexes: 2D indexes for dealing with flat data and 2D sphere indices that handle spherical information in a way similar to the earth's surface.


Here collection_name is the name of your collection.

location_field is the field name on which the index is created.  

Wildcard index

Wildcard indexes enable the querying of fields that have an unknown name or a constantly changing schema. They facilitate the execution of searches on arbitrarily defined user data structures, providing versatility and adaptability in retrieving data.

Creating a Wildcard index on a single field.

Syntax :  

Creating wildcard index on all fields.


Here collection_name is the name of your collection.

{ '$' } is the index key specification, where '$' is a wildcard character indicating that all fields within a document should be indexed.

In sort order 1 or -1 in the index key specification depicts ascending and descending order respectively.  

Hashed Index    

In MongoDB, a hashed index is a unique type of indexing that utilizes the hash value derived from a specific field to distribute all the documents efficiently across numerous shards. It's an excellent technique for uniformly spreading out data and enhancing overall system performance.


Here collection_name is the name of your collection.

The field is the field name on which the index is created.

Properties of MongoDB indexes  

Unique indexes

Unique indexes prohibit the addition of duplicate values to indexed areas. This serves to guarantee data precision and prevent inconsistencies from arising.


Sparse index  

Sparse indexes improve query performance by excluding non-indexed documents, thus optimizing index scans.


TTL index  

TTL indexes facilitate the automatic expiration of documents in a collection based on a predetermined period, typically used for data expiry policies such as eliminating time-bound information.


Case Insensitive Index  

Case-insensitive indexes allow string values to be compared without regard for capitalization. These indexes prove beneficial during sorting processes and when conducting searches that do not differentiate between uppercase and lowercase characters.


Hidden Indexes

A hidden index within MongoDB remains undetected by the query optimizer to allow flexibility in testing and optimization for some scenarios.  


Partial Index

A partial index in MongoDB only includes documents that meet a specified filter expression. These indexes prove beneficial for optimizing queries on selected documents within a collection, which reduces the size of the index and enhances efficiency.


MongoDB Indexing: The Ideal Approach & Some Best Practices

To maximize performance and optimize index utilization in MongoDB, it is crucial to adhere to specific guidelines. Some of the best practices before indexing data in MongoDB are mentioned below.

Examine Patterns of Queries

Before developing indexes, it is crucial to examine the query patterns of your application to determine which queries are executed most frequently. This will enable you to concentrate on indexing fields that feature heavily in these queries and maximize any gains in performance.


Take into account the size of the data and the capacity of the RAM.

To prevent significant performance impacts, it is important to ensure that your indexes can fit within the available RAM. If they cannot fit, possible solutions include adding more RAM or optimizing index size through partial indexing.


Prevent Excessive Indexing.

To save disk space and reduce maintenance overhead, it is important to limit the creation of unnecessary indexes. Before creating an index, assess how much benefit it will provide concerning its cost. Only invest resources into indexes that can greatly enhance the performance of the query.


Ensure to monitor the usage of indexes regularly.  

To optimize performance, keep track of your indexes' usage and efficacy through tools such as the MongoDB profiler or explain() method. Find any underutilized or unused indexes by doing so and decide whether to modify them for better efficiency or drop them altogether.


Enhance indexing for efficient write operations.

It is important to consider that adding or altering indexes can result in increased workload during write operations because every time a document is inserted, updated, or deleted the relevant index must also be modified. Therefore it's essential to find a balance between query performance and any negative effects on write performance when implementing or adjusting your indexing strategy.


Indexing plays a vital role in optimizing query performance and improving overall database efficiency in MongoDB. By creating appropriate indexes and following best practices, you can significantly enhance the speed and efficiency of data retrieval operations.

By understanding all the indexes and their characteristics, you can make informed decisions about which indexes to create and how to optimize their usage. Remember to regularly monitor and fine-tune your indexes to ensure optimal performance as your application and data evolve.

Frequently asked questions (FAQs) - What is Indexing in MongoDB?

What is the use of indexing in MongoDB? Indexing plays a crucial role in ensuring the efficient retrieval of data, thus greatly affecting MongoDB's performance. They enable swift access to relevant documents beneficial for sorting and searching tasks.  

What is an index in a database? Indexes in relational databases are special data structures designed for efficient retrieval of information. It enhances query performance considerably, especially with complex operations that include filtering or sorting.  

What is the difference between index and primary key? Indexes serve as efficient guides to locate data swiftly, hastening specific types of queries while primary keys identify each record within a table uniquely and ensure unduplicated information while maintaining integrity.  

What are the different types of MongoDB indexes? All the index types are mentioned below :

  • Single Field Index: Index created on a single field for optimizing queries based on that field.
  • Compound Index: Index created on multiple fields to optimize queries that involve multiple criteria.
  • Multikey Index: Index created on fields with arrays, generating separate index entries for each array element.
  • Text Index: Index designed for efficient text search operations.
  • Geospatial Index: Indexes tailored for queries involving geospatial coordinate data.
  • Wildcard Index:Dynamic indexes that automatically index all fields within a document.
  • Sparse Index:Indexes that omit documents that lack the indexed field are useful for optimizing storage and performance.
  • TTL Index:Time-to-live indexes automatically expire documents after a specified time, useful for managing data with a predefined lifespan.


What are MongoDB indexing best practices? Some of the indexing practices in MongoDB are:

  • Prevent Excessive Indexing.
  • Ensure to monitor the usage of indexes regularly.
  • Enhance indexing for efficient write operations.


How to get indexes in MongoDB? MongoDB’s getIndexes() function facilitates convenient retrieval of information regarding the indexes present in a collection.  

What is a namespace in MongoDB? A namespace in MongoDB consists of a concatenation between the name of a database and that of its collection (or index) separated by a period (.). Acting as an exclusive identifier, it assigns recognition to particular indices or collections within databases.  

What are MongoDB's compound indexes? A compound index in MongoDB is created when multiple fields from a collection are included. Such an index can significantly improve the performance of queries that have multiple conditions.  

What is a collection in MongoDB? A collection in MongoDB is a cluster of a number of documents stored together, serving as an equivalent to a table found in classical relational database management systems (RDBMS).  

How to create an index in MongoDB? Creating indexed fields in MongoDB can enhance the speed of query execution and enable faster data retrieval. In MongoDB Shell, use the createIndex() method to create an index.  

Written by
Rupal Sharma


What is Indexing in MongoDB: A Comprehensive Guide