This Postgres 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.

In addition to the algorithm proposed in the original paper, this implementation is augmented to improve its accuracy and memory use without sacrificing much speed.

The full description can be found at https://github.com/aggregateknowledge/postgresql-hll/blob/master/README.markdown