MongoDB Versus PostgreSQL for Transactional Systems
The honest answer for most financial or transactional systems is that PostgreSQL is the safer default. MongoDB does support multi-document transactions, but the aggregation pipeline and application code end up doing work that a relational database's joins and constraints would handle at the schema level, which increases the surface area for reconciliation bugs in systems where every number has to add up exactly.
We recommend MongoDB for these systems only when a specific part of the data, an audit log, a document store, or unstructured event data, is genuinely document-shaped, and even then we frequently keep the core financial ledger in Postgres and use MongoDB alongside it rather than as the system of record for money.








