HyperLogLog extension for PostgreSQL

 This PostgreSQL module introduces a new data type `hll` which is a HyperLogLog
 data structure. HyperLogLog is a fixed-size, set-like structure used for
 distinct value counting with tunable precision. For example, in 1280 bytes
 `hll` can estimate the count of tens of billions of distinct values with only
 a few percent error.