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How do you count distinct values at scale (HyperLogLog and approximate aggregation)?

Exact COUNT(DISTINCT) over billions of rows tracks every unique value, so it gets 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 Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Exact COUNT(DISTINCT) over billions of rows tracks every unique value, so it gets slow and expensive. The signal is knowing when a sketch like HyperLogLog buys huge memory and speed wins for a sub-percent error budget.

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