What is "hyperloglog"

HyperLogLog is an algorithm for the count-distinct problem, approximating the number of distinct elements in a multiset.

Calculating the exact cardinality of a multiset requires an amount of memory proportional to the cardinality, which is impractical for very large data sets. Probabilistic cardinality estimators, such as the HyperLogLog algorithm, use significantly less memory than this, at the cost of obtaining only an approximation of the cardinality. The HyperLogLog algorithm is able to estimate cardinalities of  > 10 with a typical error rate of 2%, using 1.5kB of memory. HyperLogLog is an extension of the earlier LogLog algorithm.