Summary

Add an approximate count distinct accumulator.

Motivation

Exact count distinct (PM-1936) is an important analytical tool but it is computationally expensive (see Appendix in here). Therefore, users often prefer to use an approximate statistic that can be applied to large volumes of data. This effort will also pave the way to support approximate count distinct in on-demand materialized views, as the exact count distinct accumulator cannot easily be incremented.