26. dubna 2019

DAX for SSAS Multidimensional

One of my major developer pains using Power BI is inability to create report level calculations in Power BI reports live connected to SQL Server Analysis Services Multidimensional. This affects my development for 2+ years and after trying to persuade product team as MVP with no success I decided to go public to support my cause and hopefully fix the problem for me and others by setting up Power BI Idea and let it vote up by public.
First, I would like to bring little bit of context. Currently I work on a project that started prior I joined the company approximately 5 years ago. Solution was built in SSAS multidimensional and is still extended even now. From SSAS we’re reporting using different technologies on Microsoft platform SSRS, ad hoc analysis in Excel and Live Connected Power BI reports.
Why live connection to cube?
Because of data size and shared business logic, that can be used in different reporting tools. Specifically measure definitions for Measures, Time related calculations, Time and Target Variances etc. Security model with Row Level Security. I don’t want to duplicate this logic in multiple data models using data import, because it would be a huge pain to maintain if logic changes for any of these. I need central definition, this is top to bottom BI Delivery.
Why not (yet) tabular?
As stated above, project started 5 years ago and Tabular by the time wasn’t there where it is now (not as Multidimensional). I agree, there were great improvements in Tabular in last 5 years and if I would stand in front of new project, I would probably take it as my first choice. But... The solution is already built. One of main motivators were time calculations stored as utility dimension (one definition working for all measures) and I know this is coming to SQL Server 2019, but I must use what I have in production now.
Truth is there is no straight forward way to migrate Multidimensional solution to Tabular without months of developer resources which we don’t have available, because new business requirements are coming. Even if we had time and resources, current solution contains a lot of logic in calculate script and I’m not 100% sure how I would model it in Tabular without refactoring DWH layer as well.
I’m sure there are other companies/consultants doing Top to Bottom BI Delivery and are stuck now with multidimensional. Please prove me right :-)
Why DAX doesn’t work against Multidimensional cubes?
And that is the point. DAX works against Multidimensional. Thanks to BISM (Business Intelligence Semantic Model) you can query Tabular and Multidimensional using MDX and you can query them by DAX as well. I’m personally using MDX against Tabular data models a lot when developing SSRS reports. And I know I can do it the other way around. It works.
So why it doesn’t work in Power BI?
Tabular works on totally different principals then Multidimensional. Multidimensional doesn’t understand concept of table and relationships as tabular interprets them. So totally different semantics of languages. Some DAX functions won’t work for Multidimensional as tabular. So Power BI team is blocking the functionality of DAX measures against SSAS Multidimensional on purpose from the beginning and had to implement UI changes to achieve that.
Problems with Power BI live connected to SSAS MD
I won’t cover here all the problems I have, just few examples. Even though formats work alright in Excel when connected to cube (Excel is client issuing MDX queries), measure is formatted in Excel correctly, but not in some Power BI visuals. Have a look at this percentage example (which is still valid by the way) https://community.powerbi.com/t5/Desktop/SSAS-Live-Connection-Formatting-Issue/td-p/413419
Chart displays numbers as decimal, data preview table correctly as percentage. This doesn’t happen every time and that is why it is painful. If formatting works or not can be impacted by logic inside of calculate script.
Second for multidimensional live connected models you can’t +/- expand matrix visual.
And the main one for me is, you can’t create measures and this is what this whole blog post is about.
If I need something simple like “Sales Ex Vat in Thousands” and I have in my cube Measure Sales Ex Vat only. I must create calculated member in cube (enterprise data model), divide the number there by 1000 for sake of one Power BI report.
This is wrong!
What would help?
Ability to create report level DAX measures.
I could create [Sales Amount Ths] = [Sales Amount] / 1000
If I have problem with [Margin %], I could create measure [Margin %_] = [Margin %] and format it on report level.
If I need to create variance [Actual]-[Target], I could do it without side effects.
Why not?
If I would need to write complex DAX, with function that could potentially not work or provide incorrect results (which is the assumed concern here). It is my responsibility as developer to dev and test the functionality.
If intellisense won’t work here, I don’t mind. I can write the formula outside of Power BI Desktop, copy and paste.
Even if it is feature which needs to be explicitly enabled to avoid misuse by random user, I’m happy with that.
Some functionality is better than no functionality.
Changing enterprise data models for sake of one report if the calculation could be derived from data in the report is wrong.

If you agree with me. If you miss the functionality. If you would like to help me from my pain. Please support this idea and share

Thank you

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