[TIR] HyperLogLogs in Redis2016年8月17日星期三
A HyperLogLog is a probabilistic data structure used to count unique values — or as it’s referred to in mathematics: calculating the cardinality of a set.
These values can be anything: for example, IP addresses for the visitors of a website, search terms, or email addresses.
Counting unique values with exact precision requires an amount of memory proportional to the number of unique values. The reason for this is that there is no way of determining if a value has already been seen other than by comparing it to the previously seen values.
Since memory is a limited resource, doing this becomes problematic when working with large sets of values.