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Redis Essentials by Hugo Lopes Tavares, Maxwell Dayvson Da Silva

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HyperLogLogs

A HyperLogLog is not actually a real data type in Redis. Conceptually, a HyperLogLog is an algorithm that uses randomization in order to provide a very good approximation of the number of unique elements that exist in a Set. It is fascinating because it only runs in O(1), constant time, and uses a very small amount of memory—up to 12 kB of memory per key. Although technically a HyperLogLog is not a real data type, we are going to consider it as one because Redis provides specific commands to manipulate Strings in order to calculate the cardinality of a set using the HyperLogLog algorithm.

The HyperLogLog algorithm is probabilistic, which means that it does not ensure 100 percent accuracy. The Redis implementation of the HyperLogLog ...

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