Postgres and Fauna are relational operational databases that serve similar use cases in modern applications, with both having relational features such as serializable transactions, database normalization, foreign keys, indexes, constraints, stored procedures, and other typical relational features. However, they differ significantly in their architecture, query language support, deployment model, and consistency properties. Postgres is based on the tabular SQL data model and supports SQL as a query language, while Fauna is based on schemaless documents and supports FQL. Postgres has richer query feature sets with custom extensions to the SQL standard but is not directly compatible with other SQL databases. Fauna's query paradigm is more operational and procedural, allowing for customization of an operational data API via procedural programming. The two databases also differ in their index implementations, consistency models, security features, replication methods, scalability approaches, deployment options, and testing methodologies. Postgres is open source, while Fauna is proprietary, with the latter offering a simpler and more consistent user experience, abstracting away many operational decisions and providing better support for modern development patterns and serverless databases.