How to distinguish a relational database PostgreSQL from a document-oriented MongoDB by architecture and principles of operation?

PostgreSQL is a relational database that stores data in tables with a fixed schema (rows and columns). It uses SQL for queries and supports transactions, relationships between tables (via foreign keys), as well as complex data operations.

MongoDB is a document-oriented database where data is stored as JSON-like documents (BSON). It does not require a fixed schema, providing flexibility in data structure. Queries are executed through its own query language, oriented towards documents.

Key differences:

• Schema: PostgreSQL — strict schema, MongoDB — flexible schema.
• Data structure: tables vs documents.
• Transactions: PostgreSQL supports complex transactions, MongoDB — limited transactions.
• Scalability: MongoDB is easier to scale horizontally.

Thus, the choice depends on the requirements for data structure and the type of operations.