adding new column from SSAS model

This is because the SSAS data import feature can generate inefficient MDX when importing. A workaround is to write your own custom query, preferably using DAX.

Once you do this, I promise you'll never go back 🙂

Here's an example query you can tweak:

DEFINE
    VAR vFilter1 =
        FILTER ( ALL ( DimProduct[Color] ), [Color] IN { "Red", "Blue" } )
    VAR vFilter2 =
        FILTER (
            ALL ( DimGeography[CountryRegionName] ),
            [CountryRegionName] = "United States"
        )
EVALUATE
SUMMARIZECOLUMNS (
    DimProduct[Color],
    DimProduct[Size],
    DimDate[CalendarYear],
    vFilter1,
    vFilter2,
    "Total Sales", SUM ( FactInternetSales[Sales Amount] ),
    "Distinct Customers", DISTINCTCOUNT ( FactInternetSales[CustomerKey] )
)