What is the memory cost of HyperLogLog?
Fixed at 12 KB regardless of cardinality. It maintains 16,384 registers (one per possible 14-bit hash prefix), each storing the maximum leading-zero count observed for that prefix. This tiny footprint works from millions to billions of unique items.
Answered in
Count Billions in 12 KilobytesHyperLogLog estimates cardinality by reading leading zeros in hashed values, trading 1% error for fixed memory.
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