How do vector databases differ from relational databases?

Vector databases and relational databases serve fundamentally different purposes due to their data models and use cases. Relational databases store structured data in tables with predefined schemas, using rows and columns to represent entities and their relationships. They excel at handling transactional data and complex queries that require joins, filters, or aggregations. For example, a relational database might manage an e-commerce platform’s inventory, tracking products, orders, and customers through tables linked by foreign keys. In contrast, vector databases are optimized for storing and querying high-dimensional vectors—arrays of numbers representing embeddings from machine learning models. These vectors capture semantic or contextual relationships, enabling similarity searches (e.g., finding images or text similar to a query). A vector database might power a recommendation system by comparing user preferences encoded as vectors to product embeddings.

The core difference lies in how they handle queries. Relational databases rely on exact matches, range filters, or joins using SQL. Queries return precise results based on strict conditions, like fetching all users with a specific ZIP code. Vector databases, however, focus on approximate nearest neighbor (ANN) searches, which prioritize speed and scalability over exact precision. For instance, searching for “images similar to this photo” involves calculating distances (e.g., cosine similarity) between vectors to find the closest matches. Relational databases lack native support for these operations, making vector databases indispensable for AI-driven applications like semantic search or anomaly detection. Additionally, vector databases often use specialized indexing techniques (e.g., HNSW or IVF) to accelerate similarity searches, whereas relational databases rely on B-tree or hash indexes optimized for structured data.

Under the hood, the two systems differ in storage and scalability. Relational databases enforce ACID compliance, ensuring data integrity for transactions, but this can limit horizontal scaling. Vector databases prioritize throughput for read-heavy workloads, often sacrificing strict consistency for performance. For example, a vector database might shard data across nodes to handle billions of embeddings, while a relational system would struggle with high-dimensional data due to indexing overhead. Developers choosing between them should consider the data type: structured, transactional data fits relational databases, while unstructured data requiring semantic analysis demands a vector database. Hybrid systems are emerging, but understanding these core differences ensures the right tool is selected for the task.