PostgreSQL vs MongoDB: choosing a database for a B2B product

The eternal argument

Startups often pick MongoDB because "it is modern and you don't need migrations". Six months later they discover relationships, reporting requirements and money movements, and start writing joins by hand in application code.

When PostgreSQL is the right answer

Postgres is the Swiss army knife: in most B2B products it is the correct default.

• You need strict consistency — billing, payments, inventory, anything where a half-written record is a business incident.
• You need complex joins and analytics that should not be recomputed in application code.
• You want flexible documents too: the JSONB type covers semi-structured data without giving up transactions.

When MongoDB earns its place

• A huge stream of unstructured data — logs, IoT readings, event payloads.
• A schema that is genuinely still changing every week in early product discovery.
• A need for cheap horizontal sharding across regions.

How we decide on a project

We look at three things: the shape of the data (does it have invariants that must always hold?), the queries (do reports need joins across entities?), and the team (who will operate this at 2 a.m.?). In practice that conversation ends with Postgres for the transactional core and, when needed, a document or search store next to it for specific access patterns.