Maybe 10 years ago I interviewed both for Netflix and Facebook.

At Netflix, I used a state machine library to handle the project they gave me. Got rejected because I didn't show I knew raw JavaScript enough.

At Facebook, I wrote a calendar dropdown from scratch. Got rejected because I should have used a library.

Interviews sometimes is just a lottery...

Yes I know both companies should have set the expectation, but you can set the expectation for EVERYTHING, otherwise you give candidates all the answers you're expecting. There's always going to be some chaos due to the huge number of variables.

It's both.

As a hiring manager I've had situations like this arise because there was a gap in my plan and I didn't realize it. When those come up, we thank them for their cleverness, apologize to the candidate, reframe the situation, and give them another shot.

But also sometimes I leave intentional ambiguity in the plan. Part of the goal is to see if they have a degree of common sense commensurate to their level. If they're interviewing for a high level position, I'd expect them to be able to spot silly flaws and push back that perhaps the whole problem needs rethinking. And of course, I also expect them to know the brute force solution as well. Do they only know one? Both? Let's fine out.

Postgres might have been a perfect answer, but the candidate needs to explain why and how.

The purpose of the interview is for the candidate to demonstrate their thought process and ability to communicate it. “Just use Postgres” doesn’t do that.

This would be more obvious if it was a LeetCode problem and the candidate just regurgitated an algorithm from memory without explaining anything about it. Yeah it’s technically the right answer but the interviewer can’t tell if you actually understand what you’re talking about or if you just happened to memorize and answer that works.

Interviews are about communication and demonstrating thought process.

The point is that you’re supposed to kiss clown’s ring and do the dance.