Postgres or MongoDB for the store? Show me the trade-off, not the answer.
Answer it out loud before you open anything. The value of the flags below is in comparing them to what you actually said — including whether you asked who uses it before naming what you would build it with.
The situation behind the question
A tech lead has to choose the store's primary database this week and asks two engineers for a recommendation. One says Postgres, one says MongoDB, both with conviction. The interviewer asks how you would settle it without a fight.
React to this
Say what you would question, what you would keep, and what you would need to know first.
The decomposition, the proposal, or the transcript as it stands
Two engineers' arguments, from the thread (illustrative)
A: "Postgres. Orders are relational. We know SQL. Transactions."
B: "Mongo. Product catalogue is document-shaped, schema will change
weekly, and it scales horizontally. Postgres is legacy thinking."
A: "Benchmark says Postgres is faster for our joins."
B: "Benchmark says Mongo is faster for our writes."
Not in the thread: what the store's invariants are, who operates the
database, whether anyone has priced switching after orders exist, or
what workload either benchmark ran.What it is really testing
Whether the candidate can turn a technology argument into a trade-off across named dimensions, avoid fake precision, separate reversible from irreversible decisions, and decide under uncertainty with a record of why.