What are the advantages of document databases over relational databases?

Document databases offer several advantages over relational databases, particularly in scenarios requiring flexibility, hierarchical data handling, and horizontal scalability. These benefits stem from fundamental differences in data modeling and storage approaches, making document databases a better fit for certain types of applications.

First, document databases provide schema flexibility, which allows developers to store data without a predefined structure. In a relational database, tables require fixed schemas, and altering columns can be time-consuming, especially in large datasets. Document databases, such as MongoDB or Couchbase, store data as JSON-like documents, enabling fields to vary between records. For example, an e-commerce product catalog might include items with different attributes (e.g., weight for physical goods, download size for digital products). A document database can accommodate these variations without requiring schema migrations or complex joins across tables. This flexibility accelerates development cycles and simplifies handling evolving data requirements.

Second, document databases excel at managing hierarchical or nested data. Relational databases often require normalization, splitting data into multiple tables and using joins to reconstruct relationships. Document databases store related data as nested objects within a single document. For instance, a user profile with addresses, preferences, and order history can be represented as one document, reducing the need for complex queries. This approach improves read performance for hierarchical data, as retrieving a complete entity requires a single database operation. While relational databases can handle such scenarios with JSON columns, they lack the native querying capabilities and indexing optimizations that document databases provide for nested structures.

Finally, document databases are often designed for horizontal scalability, making them suitable for large-scale applications. Relational databases typically scale vertically (upgrading server hardware), which has physical and cost limitations. Document databases like MongoDB support automatic sharding, distributing data across multiple servers to handle high traffic or large datasets. For example, a social media app storing millions of user posts with varying metadata can scale horizontally by sharding data based on user regions or activity patterns. While relational databases can achieve horizontal scaling through techniques like read replicas, document databases simplify this process with built-in distributed architecture, reducing operational complexity for developers managing growth.