System InternalsLearn ↗HyperLogLogADDESTIMATE

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The estimate landing close to 300 from just 64 small registers — no list of the actual values is ever stored.

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How HyperLogLog estimates how many unique items it has seen using a fixed, tiny amount of memory.

How it runs

Each value is hashed; the leading zeros of the hash pick how 'rare' it is, and each register keeps the rarest it has seen. Combining all registers with a harmonic mean turns those rarity records into a count estimate.