This is just an indication that it's a poor interviewing technique. If you ask a question expecting the answer to be predictable then you had better cover all possible ambiguity in leading the candidate to the answer you want to hear. But then you're no longer asking the candidate to be creative, are you? As an interviewer I might be more inclined to allow such an answer and then follow up with questions of my own to test the limits of the candidate's knowledge of and confidence in Postgres as a technical choice for serving all the different constraints of the problem space. This way either I discover how well-reasoned the answer is or else (for a good candidate) it prompts them to adjust the design to better fit the problem. In no case would I expect they need to fit their answer into some key and then scold them for not playing along with my imaginary game.