Add a way to specify general grouping sets and special ones like rollup and cube

[lukaseder]

It would be nice to implement accumulators / collectors that implement SQL's GROUPING SET semantics by duplicating data into possible grouping set permutations.

Possibly, this is only feasible via #105

[tkroman]

Hey @lukaseder,
can you provide an example/specification on this one?

[lukaseder]

Interested in contributing? :)

GROUPING SETS are a very SQL OLAP operator. The following SQL Server documentation page explains nicely, how they work:
https://technet.microsoft.com/en-us/library/bb522495(v=sql.105).aspx

In other words, they're combining GROUP BY and UNION ALL operations in a concise way, which is useful specifically with aggregations.

It may well be that this isn't really possible in a sensible way in functional programming....