Rather than be annoyed, you'd probably get bonus marks if you could show him the group-by query (sounds like you could have done that easily), then explain why your way is better. The interviewer would have been impressed at your maturity and skill.
For your information, this was the later stage of questions. The very first question did involve a simple summation group by, so I'd already demonstrated my understanding there. I showed no frustration or annoyance during the interview in case you are attempting to imply that was the case.
wait, what about the GIL?!? did they fix that?