PostgreSQL vs. MongoDB for JSON: The Internal Trade-offs They Don't Tell You in the Docs

The question comes up constantly in architecture discussions: "Should we use MongoDB or PostgreSQL for our JSON-heavy workload?" Having managed both at scale, I can tell you the answer is not as simple as "MongoDB is for documents, Postgres is for tables." There are deep internals at play on both sides that will affect your performance, storage footprint, and operational burden in ways that a quick benchmark won't reveal. Let's dig in. First, a Fundamental Framing Problem MongoDB is often called a "document database," which people interpret as: great for JSON, superior to relational databases for flexible data. That framing is misleading. MongoDB is not simply a JSON store with a query layer on top. It is a non-relational database, meaning it has no native concept of joins, no foreign key enforcement, no referential integrity, and no support for multi-document ACID transactions that span arbitrary collections (multi-document transactions were added in v4.0 but carry significant perform