[Mondrian] Adding Grouping Set support for Distinct Count measures

[John V. Sichi]

Ajit Vasudeo Joglekar wrote:
> 
> The current logic can certainly be optimized for better performance. 
> Could you please share the mdxs that were taking especially longer time? 
> We can start looking into this

This is the one I used for comparison (intentionally pathological to 
make it easy to spot the problem while profiling).  Customer Count 
provides the necessary DISTINCTCOUNT.

With
Set [*BASE_MEMBERS_Customer] as [Customers].[Name].members
Set [*BASE_MEMBERS_Store] as [Store].[Store Name].members
Set [*BASE_MEMBERS_Product] as [Product].[All Products].[Food].children
Member [Customers].[*SUBTOTAL] As Aggregate([*BASE_MEMBERS_Customer])
Member [Product].[*SUBTOTAL] As Aggregate([*BASE_MEMBERS_Product])
Member [Store].[*SUBTOTAL] As Aggregate([*BASE_MEMBERS_Store])
Select
{[Measures].[Customer Count]} on columns,
([Store].[*SUBTOTAL],[Customers].[*SUBTOTAL],[Product].[*SUBTOTAL])
on rows
  From [Sales]
where ([Time].[1997]);

JVS