"Relational databases are great ... after the problem set has been well defined ... But a lot of times that information isn't obvious upfront, and mongo is grea

"Relational databases are great ... after the problem set has been well defined ... But a lot of times that information isn't obvious upfront, and mongo is great in that context." A lot of people get stuck on the schema because they are trying to make it perfect upfront and hit analysis paralysis. I think that comes from other database systems that make it very hard to change the schema. In postgres, just define a very simple schema and get started with your prototype. It may just have a couple ordinary columns, and then a JSON or JSONB column as a catchall. Then, you can add/remove/rename columns and tables (all O(1) operations) or do more sophisticated transformations/migrations. All of this is transactional, meaning that your app can still be running without seeing an inconsistent (half-migrated) state. I believe this level of flexibility far exceeds what document databases offer.