Excel Multiselect Issues with SSAS Calculated Member

Ok, it’s an old one, but I struggled to find the right answer, so I thought it was worth a post. I knew it was an issue, and I thought I knew the answer. I was wrong.

Say you create a calculated member to return some kind of item count against a dimension, a bit like this:

CREATE MEMBER CURRENTCUBE.[Measures].[Days Count] as (
count( descendants(
  [Date].[Year-Month-Date].CurrentMember,
  [Date].[Year-Month-Date].[Date]
))

and it works just fine when you test it

SELECT {
    [Measures].[Days Count]
    ,[Measures].[Minutes Of Day Count]
} ON COLUMNS
FROM [My Cube]
WHERE {
    [Date].[Year-Month-Date].[Year].[2011]
}

…and then you find it falls apart when someone queries the cube in Excel and uses multi-select in the pivot table filter. So you follow Mosha’s advice to fix it (using Existing):

CREATE MEMBER CURRENTCUBE.[Measures].[Days Count] as
count( existing [Date].[Year-Month-Date].[Date] )

…but then someone points out that that doesn’t work either. More recent versions of Excel (2007, 2010) implement multi-select via sub-queries; with just a few days selected it generates a query that (simplified) looks a bit like this:

SELECT {
    [Measures].[Days Count]
    ,[Measures].[Minutes Of Day Count]
} ON COLUMNS
FROM (
    SELECT (
        {[Date].[Year-Month-Date].[Date].&[20110101]
        ,[Date].[Year-Month-Date].[Date].&[20110102]
        })
    ON COLUMNS  FROM [My Cube]
)


and executing that doesn’t give you the right answer (it just counts the number of members in the dimension):

So the old advice, still commonly quoted, is actually wrong. The solution (as of SSAS 2008 and onwards) is to use a dynamic named set:

CREATE DYNAMIC SET [Selected Days] as (
    [Date].[Year-Month-Date].[Date]
)
CREATE MEMBER CURRENTCUBE.[Measures].[Days Count] as (
    count([Selected Days])
)

I found this a bit bizarre at first, but some of the posts I read were themselves a bit confused and left in the ‘existing’. This is not required, and distracts from what’s really going on here: this works by design as this is one of the problems that dynamic sets are intended to fix. A (more recent) Mosha post spells it out:

“Dynamic sets are not calculated once. They are calculated before each query, and, very important, in the context of that's query WHERE clause and subselects”

Ah. The answer, and the explanation. Finally.