Support for HyperLogLog aggregate functions

HyperLogLog is an algorithm/structure by which cardinality of a set may be estimated. It is much more efficient than count(distinct x) when a degree of error is permissible and can be used in warehousing applications, where multiple estimates of set cardinality can be merged to produce an estimate of the set union.

See blog.aggregateknowledge.com for a lot of discussion of the algorithm and for an PostgreSQL based implementation similar to what would be suggested for inclusion in Maria.

http://blog.aggregateknowledge.com/2013/02/04/open-source-release-postgresql-hll/