Non-psychic candidates will struggle with this challenge, so you might find it produces some false negatives.
Since you already seem to know so much about debugging slow performing database queries on your particular RDBMS stack, though... what was your thinking in looking to bring on board someone else who duplicates that exact knowledge?
Oh, and it'll be a missing index. It's always a missing index.
The point is to start with something open ended, so the candidate can talk as much as they want on the subject, broadly, and then, based on what they say and what they don't say, I narrow in on particulars:
"Are you aware of any differences in dialect between the SQL of MySQL and the SQL of PostGres?"
It's not like they lost points for the stuff they didn't know, I simply wanted to be sure I understood the limits of their knowledge. We hired this candidate and they turned out to be great. But I hired them knowing exactly how much they knew and how much they did know.
You might want to consider if there’s a way to discover that without triggering the candidate’s imposter syndrome. Your script above reads like Tom Cruise trying to get Colonel Jessup to admit he ordered the code red. All that’s missing is the candidate breaking at the end and screaming “YOU’RE DAMN RIGHT I DON’T KNOW HOW TO DEBUG SQL”.
That is exactly what I'm going for. I'm actually looking for 2 things:
* If they don't know how to debug SQL, I'd like to know that.
* Do they simply say they don't know, or do they try to bluff? Are they confident about what they don't know? Like I said in a different comment in this thread, one of my greatest hires ever was a former teacher who was very confident about what she didn't know -- she never felt any need to bluff.
Given two people of exactly equal skill, if one bluffs and the other is confident enough to say "I don't know", then I'll hire the one who says "I don't know." Because then when we are actually working together, I know they will be honest with me whenever they need help, they won't be trying to keep secrets from me.
Also, about this:
"triggering the candidate’s imposter syndrome"
Given two people of exactly equal skill, if one has imposter syndrome and another doesn't, I'll take the one who doesn't have the imposter syndrome. Or put differently, given two people of exactly equal technical skill, if one demonstrates a higher emotional intelligence, I'll hire that person.
If they admit they don't know how to debug SQL in an interview, that is actually a positive thing. They know their current limitations and are honest about it. If they start looking for problems in irrelevant areas (like HTML) and making things up when they were specifically told the problem was elsewhere (backend), that is a bad sign.
Non-psychic candidates will struggle with this challenge, so you might find it produces some false negatives.
Since you already seem to know so much about debugging slow performing database queries on your particular RDBMS stack, though... what was your thinking in looking to bring on board someone else who duplicates that exact knowledge?
Oh, and it'll be a missing index. It's always a missing index.