Denormalization of data in MongoDB

MongoDB, being a NoSQL document-oriented database, offers a flexible schema design. One of the key design strategies in MongoDB is denormalization, where related data is stored together in a single document rather than being split across multiple collections. This article explores denormalization in MongoDB, its advantages, disadvantages, use cases, and best practices. Additionally, we will provide MongoDB queries with detailed explanations and outputs.

What is Denormalization?

Denormalization is the process of optimizing database read performance by embedding related data into a single document, reducing the need for complex joins or multiple queries. Unlike relational databases where normalization is used to remove redundancy, MongoDB encourages denormalization for better performance.

Denormalization in MongoDB can be done using:

• Embedded Documents – Storing related data directly within a document.
• Referencing with Redundancy – Keeping references to related data while storing frequently needed fields.

Example of Implementing Denormalization in MongoDB

To understand denormalization better, let's consider an e-commerce database with users and their orders.

1. Normalized Approach (Using References)

In a normalized schema, we store user and order details in separate collections.

User Collection (users):

{
  "_id": 1,
  "name": "Alice",
  "email": "[email protected]",
  "orders": [101, 102]  // Order IDs stored as references
}

Orders Collection (orders):

{
  "_id": 101,
  "user_id": 1,
  "order_date": "2024-02-18",
  "total": 150
}
{
  "_id": 102,
  "user_id": 1,
  "order_date": "2024-02-19",
  "total": 200
}

Query to Retrieve User Orders (Using $lookup):

db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      localField: "orders",
      foreignField: "_id",
      as: "order_details"
    }
  }
])

Output:

[
  {
    "_id": 1,
    "name": "Alice",
    "email": "[email protected]",
    "orders": [101, 102],
    "order_details": [
      {
        "_id": 101,
        "user_id": 1,
        "order_date": "2024-02-18",
        "total": 150
      },
      {
        "_id": 102,
        "user_id": 1,
        "order_date": "2024-02-19",
        "total": 200
      }
    ]
  }
]

Explanation:

• We use the $lookup stage in the aggregation pipeline to join users with orders using the order IDs.
• The query results in an array of orders inside the user document.
• This approach is similar to SQL JOIN, but it requires additional processing time.

2. Denormalized Approach (Using Embedded Documents)

Instead of storing orders in a separate collection, we embed them within the users collection.

Denormalized users Collection:

{
  "_id": 1,
  "name": "Alice",
  "email": "[email protected]",
  "orders": [
    {
      "order_id": 101,
      "order_date": "2024-02-18",
      "total": 150
    },
    {
      "order_id": 102,
      "order_date": "2024-02-19",
      "total": 200
    }
  ]
}

Query to Retrieve User Orders:

db.users.find({ _id: 1 })

Output:

[
  {
    "_id": 1,
    "name": "Alice",
    "email": "[email protected]",
    "orders": [
      {
        "order_id": 101,
        "order_date": "2024-02-18",
        "total": 150
      },
      {
        "order_id": 102,
        "order_date": "2024-02-19",
        "total": 200
      }
    ]
  }
]

Explanation:

• This structure removes the need for a separate query or $lookup.
• Orders are directly embedded in the user document, making retrieval faster.

Advantages of Denormalization

• Faster Read Performance – Retrieves all required data in a single query, reducing lookup time.
• Reduced Query Complexity – Eliminates the need for joins ($lookup), simplifying queries.
• Optimized for Read-Heavy Applications – Ideal for e-commerce, analytics, and dashboards.
• Improved Scalability – Works efficiently in sharded clusters and large-scale deployments.
• Lower Latency – Reduces database calls, ensuring quick responses in high-traffic environments.
• Minimized Database Contention – Fewer queries compete for resources, reducing system load.
• Better Caching – Complete documents can be cached, improving performance.
• Offline Processing Efficiency – Helps with data preloading and batch operations.

Disadvantages of Denormalization

• Increased Data Redundancy – Duplicate data leads to higher storage usage.
• Complex Updates – Updating embedded data requires modifying multiple documents.
• Risk of Data Inconsistency – Partial updates can leave data in an inconsistent state.
• Document Size Limitations – MongoDB has a 16MB document limit, restricting deep nesting.
• Slower Write Operations – Inserts and updates take longer due to larger documents.
• Performance Bottlenecks for Write-Heavy Applications – Not suitable for frequently updated data.
• Complicated Indexing – Indexes need to handle larger documents, affecting performance.
• Schema Maintenance Challenges – Requires careful planning to balance performance and flexibility.

Best Practices for Denormalization

• Embed small, frequently accessed data to improve read performance.
• Avoid embedding large, frequently changing data to prevent costly updates.
• Use references with redundancy by storing only essential fields.
• Monitor document size to stay within MongoDB’s 16MB limit.
• Optimize indexing to ensure efficient query execution.
• Denormalize only where necessary to balance speed and maintainability.
• Regularly review schema design to adjust based on application needs.

Conclusion

In conclusion, denormalization in MongoDB enhances read performance by embedding related data into a single document, reducing the need for complex joins and multiple queries. This approach simplifies data retrieval, making it ideal for read-heavy applications like e-commerce and analytics. However, it also introduces challenges such as data redundancy, complex updates, and potential data inconsistency. To optimize denormalization, developers should follow best practices, like embedding small, frequently accessed data, monitoring document sizes, and optimizing indexing. While denormalization offers significant benefits in terms of speed and scalability, it requires careful consideration of the application’s needs and data patterns.