How do you count distinct values at scale (HyperLogLog and approximate aggregation)?
Exact COUNT(DISTINCT) over billions of rows has to track every unique value, so it turns slow and expensive. The signal is knowing when a sketch like HyperLogLog buys huge memory and speed wins for a sub-percent error budget.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Exact COUNT(DISTINCT) over billions of rows has to track every unique value, so it turns slow and expensive. The signal is knowing when a sketch like HyperLogLog buys huge memory and speed wins for a sub-percent error budget.
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.
