Using the HyperLogLog algorithm to estimate the number of unique elements in huge datasets: http://blog.notdot.net/2012/09/Dam-Cool-Algorithms-Cardinality-Estim

Using the HyperLogLog algorithm to estimate the number of unique elements in huge datasets: http://blog.notdot.net/2012/09/Dam-Cool-Algorithms-Cardinality-Estimation http://blog.notdot.net/2012/09/Dam-Cool-Algorithms-Cardinali... Very cool algorithm used in (among other things) redis.