What is a correlated subquery, and why can it be a performance trap?
Correlated subqueries read naturally but may run once per row (O(n squared)), a classic slow-query cause. The signal is spotting them and rewriting as joins or window functions.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Correlated subqueries read naturally but may run once per row (O(n squared)), a classic slow-query cause. The signal is spotting them and rewriting as joins or window functions.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
