Dimension formulas & SQL AS 2005

Question

I'm in the planning process right now for the upgrade from 4.2 to 5.1.  We have already had a technical with SAP and recently one with the consultants that will be assisting us with the upgrade.  One of the main concerns that our consultants had was the amount of dimension formulas that we have within our application.  They spoke about an issue with MS AS 2005 and the performance hit associated with dimension formulas.  They also said that Microsoft would not be addressing this within SQL 2005, but that there would be a fix in 2008.  That being said, I have 2 questions. 
1)  Does anyone know if there is a MS Knowledge base article that I can reference?
2)  Does anyone have any advice for where to begin addressing the dimension formulas and moving them into business rules and/or sql?

Answer 1

It has always been the case that you should avoid using MDX for formulas that cannot be calculated at all levels with the same rules.  Though I’ve seen lots of cases where people write a lot of MDX that should never have been written and could be done better in either scripting logic or spreadsheet logic.  Lots of nested IIF statements or other complex expressions will generally perform poorly.  If AS cannot use aggregations or the formulas are sufficiently complex that AS cannot determine for sure if it can use the aggregations then it will perform the necessary aggregations at query time to derive the values.  This ABSOLUTLY kills AS performance.  The major performance factor in AS is the pre-aggregations as far as query time goes.
For example you can do a Units x Price calculation in MDX and write the formula so that it will be correct at the base members and all rollups.  However, if you pick a top level member the query will run forever because AS will calculate every leaf level UxP calculation below the level you are at across the hierarchies and then aggregate all the values to derive the correct value.  I’ve seen calculations like this run for hours.  This is the entire reason that scripting logic was created to allow these types of calculations to be executed in the relational store and the results aggregated along with the other data in AS.
MDX should only be used for things such as basic reporting ratios such as Gross Margin Pct which is simply [Gross Margin]/[Sales] regardless of the level you are at in the hierarchies and will easily use any aggregations that are available at the intersection that the query is requested for.  This is true for AS 2000, 2005 and I’m sure it will be in 2008.  This has nothing to do with BPC, it’s simply the strengths and weaknesses of AS.
What we have seen is that in AS 2005 it is more prone to performance issues so greater care in how a formula is written is needed.  Also, with BPC 5.x the way multiple hierarchies was changed as discussed earlier so even Gross Margin Pct become a more complicated formula in the presence of multiple hierarchies (in the dimension that the formula is written in) just from the syntax required to qualify out the various hierarchy cases.