When I first saw a demo of Excel 2007 in the Beta 1 timeframe, I was told that the Slicers in Excel 2007 would still use the WHERE clause, not the new Subcube Subquery available in SQL2005. I was very disappointed (partly because in the beta timeframe for SQL 2005, the Where clause behaved so badly). But when I got my copy of Excel 2007 and looked at the actual MDX, it turns out that they do use the subcube subquery. At first I was happy, but then I discovered a strange consequence of using the subcube subquery (while, in a bit of irony, by RTM the relative benefit of a subcube subquery has decreased because they fixed the behavior of the Where clause).
Suppose you have a "Sales Growth" measure. In all likelihood, the MDX will include something like ([Measures].[Sales],[Time].[Calendar].CurrentMember.PrevMember). Now suppose you have a query with [Nov 2007] as the slicer. If the slicer goes into the WHERE clause, then the current member of Time.Calendar is [Nov 2007] and the prev member is [Oct 2007] and all is good. But if the slicer goes into a subcube subquery, then the Time.Calendar dimension gets collapsed to [All]-->-->[Q4]-->[Nov 2007], but the current member is [All], not [Nov 2007]. With the subquery, the value of [All] equals the value of [Nov 2007], so the basic measures look alright, but the prev member for [All] is Nothing, so the growth calculation screws up.
The only way I've been able to get the correct growth to show up is by explicitly putting the Time dimension onto the Row or Column axis, so there's a meaningful CurrentMember. It works, but it looks a bit stupid.
I was hoping that the Existing operator (that I recently discovered) would provide a workaround for the problem (because the formula could then use Existing Time.Calendar.Month.Members. But, sadly, the "current context" for the calculated member does not take into consideration the subcube subquery restriction, so you get the entire set of months, not just the "current" month. So this approach did not work.
If anyone has an idea for how to write a formula that properly calculates growth, even when the filtering is done by using a subcube subquery, I'd love to hear it.
- Reed Jacobson