What are the limitations of relational databases?

Relational databases have several key limitations that developers should consider when designing systems. The three most significant constraints involve scalability challenges, schema rigidity, and difficulties handling unstructured or hierarchical data. While relational databases excel at structured data management and transactional consistency, these limitations can make them less suitable for modern use cases requiring flexibility, horizontal scaling, or complex data relationships.

The first major limitation is horizontal scalability. Relational databases are optimized for vertical scaling (adding more power to a single server) rather than horizontal scaling (adding more servers). This becomes problematic when handling massive datasets or high write volumes. For example, a social media app with millions of daily users might struggle with a relational database because sharding (splitting data across servers) requires complex configuration and often breaks transactional guarantees. While cloud solutions like Amazon Aurora mitigate this to some extent, they still can’t match the effortless scaling of NoSQL databases like Cassandra, which distributes data automatically across nodes without sacrificing write availability.

Second, relational databases enforce rigid schema structures that complicate iterative development. Adding or modifying columns in production often requires downtime or complex migration scripts. Consider an e-commerce platform needing to add a “pre-order_date” field: In NoSQL systems like MongoDB, developers could insert this field immediately for specific products without affecting existing records. In contrast, a relational database would require altering the table schema, potentially locking the table during migration. Additionally, relational databases struggle with hierarchical data like JSON documents. While PostgreSQL supports JSONB columns, query performance and indexing efficiency for nested data still lag behind purpose-built document stores.

Finally, complex joins and transactions can degrade performance in large datasets. A reporting system analyzing relationships across 10 normalized tables might experience slow queries despite proper indexing. While denormalization helps, it introduces data redundancy and synchronization challenges. Relational databases also face difficulties in distributed environments due to their ACID compliance focus. Implementing multi-region replication while maintaining strong consistency often results in latency spikes, unlike eventually consistent NoSQL systems designed for global distribution. These trade-offs make relational databases less ideal for real-time analytics or globally distributed applications requiring millisecond response times.