PostgreSQL

PostgreSQL is the world's most advanced open-source relational database, and it has become the de facto standard database for AI applications and agent backends. PostgreSQL's extensibility — particularly the pgvector extension for vector similarity search — has made it a popular choice for developers who want a single database for both traditional data and AI embeddings.

The pgvector extension allows PostgreSQL to store and query vector embeddings alongside regular relational data, enabling RAG applications and semantic search without requiring a separate vector database. This 'just add vectors to Postgres' approach has been widely adopted by AI startups and agent platforms.

PostgreSQL's position in the agent stack is foundational: it serves as the persistent memory layer where agents store state, conversation history, user data, and vector embeddings — all in a single, battle-tested database system.