[kstrauser]

FedEx needs both big, slow, high-volume trucks for moving stuff between cities, and small, nimble tricks for delivering to the doorstep. Someone decides that it’s inefficient to maintain two separate standards: any driver should be able to get into any available vehicle and have it Just Work for whatever job they have at hand, right? So they decide to make a single vehicle that can fulfill all roles.

Well, a vehicle that can squeeze down an alley won’t have the cargo room of a giant highway truck. The inter-city drivers will hate its poor capacity. Likewise, one that has a big 20-speed transmission for for hauling heavy loads is going to drive the city drivers nuts. They’re going to end up with one single interface to all possible roadways that everyone can come together and agree to hate.

If the database API is so free-form that you can store anything in it, you won’t get the advantages of PostgreSQL’s strict typing and lightning fast joins. If you make it so regimented that your data model ends up looking like a set of tables with foreign keys, then it won’t be able to make full use of MongoDB’s... whatever it does well.

They’re different animals. Choosing one highly affects the rest of your system design, from how you arrange your data to how you add new data to how you search for it. PostgreSQL and MongoDB have fundamentally different strengths and weaknesses, and if you make something that works equally well with both, it’s inherently going to suck equally on either.