Boyan Penev on Microsoft BI

By default SSRS MDX queries get a StrToMember or StrToSet functions with a CONSTRAINED flag. However, many developers do not quite know why it is there or what it actually does. Books-On-Line contains this statements:

StrToMember

When the CONSTRAINED flag is used, the member name must be directly resolvable to a qualified or unqualified member name. This flag is used to reduce the risk of injection attacks via the specified string. If a string is provided that is not directly resolvable to a qualified or unqualified member name, the following error appears: “The restrictions imposed by the CONSTRAINED flag in the STRTOMEMBER function were violated.”

When the CONSTRAINED flag is not used, the specified member can resolve either directly to a member name or can resolve to an MDX expression that resolves to a name.

StrToSet

When the CONSTRAINED flag is used, the set specification must contain qualified or unqualified member names or a set of tuples containing qualified or unqualified member names enclosed by braces {}. This flag is used to reduce the risk of injection attacks via the specified string. If a string is provided that is not directly resolvable to qualified or unqualified member names, the following error appears: “The restrictions imposed by the CONSTRAINED flag in the STRTOSET function were violated.”

When the CONSTRAINED flag is not used, the specified set specification can resolve to a valid Multidimensional Expressions (MDX) expression that returns a set.

Therefore, if you have a CONSTRAINED flag you have to specify exact members or sets (e.g. [Date].[Year].[2009], or {[Date].[Year].[2009],[Date].[Year].[2010]}). If you omit the flag, you can pass to the StrToMember an expression, which evaluates to a member (e.g. [Date].[Year].[Year].Members.Item(0)), and to StrToSet an expression, which evaluates to a set (e.g. NONEMPTY([Date].[Year].[Year].Members, [Measures].[Amount]).

The flexibility which removing CONSTRAINED offers can be quite powerful when passing parameters between reports. In example, we may want to pass a parameter to a drill-through report from two different summary reports, where each of those uses a different subset of dimension members, which in turn can be derived by different set expressions.

Major drawbacks of using this approach is the severe performance hit it leads to, as well as a possible “MDX injection” vulnerability. Since in most cases we would  be using the passed parameters in a subcube expression or on the slicer axis (WHERE clause), this should not lead to as bad performance as we would get if we would use it inside a calculation. However, when we need to use a parameter directly in a calculated measure, we would be better off avoiding an unCONSTRAINED function. 

 Therefore, we may instead use SetToStr on the summary reports and pass a String parameter to a CONSTRAINED StrToSet function in the drill-through report. This way we are first resolving the set expression once and passing it on to the underlying report as a string. We could do that in a calculated measure returning a String, then passed on as a Field to the drill-through parameter. However, in the rare case where we have many rows travelling from the SSAS server to the SSRS server, this could be slow.

So, whether we use a StrToSet without CONSTRAINED flag, or a String parameter constructed by a SetToStr function, is dependent on the actual scenario, but it is good to have both options in our arsenal of tools and techniques when we need to implement some non-quite-standard piece of functionality.

SSAS, SSRS Analysis Services, MDX, parameters, Reporting Services

Just last weekend I implemented a number of Slowly Changing Dimensions in a SQL Server 2005 based project. For the large ones I wrote some SQL code, but for the smaller dimensions, I just decided to use the SSIS SCD task. Since the wizard does most of the work, there is not much beyond it I have done in the past with that component. This time, though we decided to have custom default EffectiveTo dates for the dimensions – 9999-12-31 instead of the default for the SCD task NULL. The wizard, however is not customisable and some manual teaks need to be done before it can handle custom dates. So, I decided to share these since there is not much around on this topic (or at least I could not find any particular references). There is a customisable component on Codeplex – Kimball Method SCD Component, however I could not use it as no custom tools could be used for an unknown reason.

I created a quick mock up of a dimension table for demonstration purposes:

Then, I created a SCD task in SSIS with one historical and one changing attribute. For Start and End dates I used my EffectiveFromDateId and EffectiveToDateId and got them populated with [System::StartTime]. Unfortunately, the SSIS task does not allow specifying custom values for the default To date, and uses NULL by default. To change it, we have to modify the following three dataflow components:

We can modify these through the Advanced Editor (right-click). For the actual Slowly Changing Dimension task we have make the following change:

Then we also have to modify the SQL script  for the two OLE DB commands (again through the advanced editor). For the Changing Attributes Updates Output:

And a similar change to the Historical Attributes Inserts Output:

After applying these three changes, we are ready to run the task:

As we have three new rows, they get inserted in the target dimension table. As they are all active, their EffectiveToDates are the default values of 9999-12-31:

Of course, if we decide to change anything through the SSIS SCD wizard, all of these will be lost and we have to redo these changes once again…

SSIS SCD, SSIS, Type 2

MDX is a great way to achieve some objectives easily and efficiently. However, there are some things better done in other ways. I will first discuss three distinct mistakes, which designers and developers tend to make when working on a typical BI solution.

1. Leaf-Level Calculations

This is by far the most common one. Both on the MSDN SQL Server forums, and in practice – developers try building calculations in MDX on the leaf level of some dimensions and usually hit severe performance problems. While it is usually possible to build an MDX expression to achieve the goal, it is usually much simpler and way better for performance to just do the calculation either in the ETL, or in the DSV (either as a Named Calculation, or as a part of a Named Query). This avoids the need for the query engine to perform a large number of calculations every time we request an aggregate.

2. Mocking Joins

I have discussed this in a previous post, where I am explaining how we can access a measure group through a dimension, which is not directly related to it (but rather related to it through another measure group and another dimension). Well, instead of doing this, we can simply build a many-to-many relationship between the two and avoid the whole MDX bit.

3. Business Logic over a large dimension

MDX is brilliant for implementing business logic. Unless it needs to operate over millions of dimension members every time a calculation is being requested. In example, recently I tried building a bit of business logic, which needed to order a dimension over a measure, and get the member with a largest value for each member of another dimension with hundreds of thousands of members. On top of it there were other calculations doing similar logic and the end result was not quite what was expected. Even though the MDX was really neat and achieved the purpose in 3-4 lines, I moved the logic back to the ETL (which was quite a bit more complex) because of performance. So, in general, I would not advise in favour of using MDX when to retrieve the result, the query engine needs to go through a lot of cells (in my case quite a few million), especially when ordering is required.

A more fundamental mistake is using Analysis Services in a solution that does not really need it. Two severe and common, in my opinion mistakes are:

1. Data Dumps

Why build a cube when the sole purpose of the project is to allow users to access the underlying data? Unfortunately, sometimes Analysis Services is seen as a silver bullet for everything. If the end report contains a massive amount of data and a key requirement is for it to export nicely to CSV, why bother – just export the data to CSV, zip it up and let the users download it. As far as I know, this can be achieved very easily in a number of other ways. Especially considering the amount of effort and skills needed to build and maintain a SSAS solution.

2. No Aggregations

Another way SSAS gets misused is when a lot of textual data gets stored in a large number of big dimensions, and those get linked in a “fact table”. I have previously worked on a solution where there were no measure columns in the fact table at all and the cube was used to retrieve information about dimension members of the largest dimension called “Member”, containing 4-5 million customers. The rest were dimensions like “Sign Up Date”, “Date Suspended”, “Country of Birth”, “Age Band”, etc. In the end, the main report consisted of the information about the members. No data was aggregated apart from a simple count. The entire OLAP solution could have been replaced by a SQL query with a WHERE clause and an index.

I am sure that there are many other cases when SSAS gets misused. A solution utilising SSAS and MDX properly can be very powerful and elegant. However, sometimes because of poor design choices it gets discarded as inadequate. Don’t blame the tools and the technology if your cube is slow – it is most likely a problem with either your design or the way you have developed your solution.

SSAS Analysis Services, MDX

I have been a little busy recently with getting certified with Microsoft. I am quite happy to announce I have passed both 70-448 and 70-452 exams in the last couple of weeks without reading a single page of preparation material. Now I am MCP, MCTS and MCITP in SQL Server 2008 BI.

I have always wondered how important and relevant these certificates are. The common opinion around the industry professionals seems to be that experience is what really counts and certifications are for people without experience who are trying to get into the profession. I did it the other way and I am not sorry the least bit for it. An overview of my experience in regards to the two certifications:

MCTS: Microsoft SQL Server 2008, Business Intelligence Development and Maintenance
Exam: 70-448 Microsoft SQL Server 2008, Business Intelligence Development and Maintenance

I passed this one with 857 (out of 1000). Since it was my first attempt at certification I did go through a few sample questions from a prep book based on MeasureUp, and a friend also flicked my way some really poor quality Braindumps, which contained some quite poor terminology and a fair bit of nonsense. On the MeasureUp tests I scored between 65-85%, so I decided to get a Free Second Shot voucher from Prometric and just go with no further preparation. As expected, my Data Mining skills did not quite cover the expectations and I had a few glitches around SSIS maintenance, but as expected I got more than 90% on all the development components and around 80% on all the administration/maintenance ones. After all I am a developer and I am not that experienced with administering BI solutions. So in general, my impression was that the test can be passed without much preparation from fairly inexperienced developers, who read some preparation materials.

MCITP: Business Intelligence Developer 2008
Exam: 70-452 PRO Designing a Business Intelligence Infrastructure Using Microsoft SQL Server 2008

This one was harder, but I passed it with 92% with doing one sample test of 30ish questions before the attempt. I had another Prometric Free Second Shot voucher, so I was not stressing if I would pass or fail. The test had double the amount of questions than the MCTS one and it took me a fir while to go through all of them. Again – Data Mining was my weakest part (less than 50% right), while I managed to score 100% on SSRS, SSAS and “Designing the BI Architecture” part. SSIS was almost perfect too. Now, if a fairly inexpreienced developer passes MCTS, this test will definitely cause a lot more headaches. The questions are much more practical and much higher degree of relevant experience would be required to pass (or much more reading).

In general, both of the tests are not easy and I do believe that if one can pass them without preparation (from experience), he is quite prepared to tackle the design, implementation and administration of Microsoft BI solutions. The Free Second Shot vouchers are also great for eliminating stress to some degree. However, on the not-so-good side, the tests can be passed with no experience and because the questions do follow a certain pattern and do not allow for much freedom in choosing the correct answer (short answer ones could be better), I think that there is a moderate chance of inexperienced and not that knowledgeable people to study, pass and then immediately forget the subject matter.

Still, I have no idea how much my career will benefit from these certifications, however I did get a few PDF certificates signed by Steven A. Ballmer to wave at sutiable occasions :)

Other certification, MCITP, MCP, MCTS, SQL Server 2008

Just last week I attended a Microsoft presentation with a subject: “SQL Server 2008 R2 for Developers”. The presenter went through most of the new features in R2 and in SQL Server 2008 but the largest chunk of the presentation was dedicated to PowerPivot. Unsurprisingly, the 50+ developers in the room had not heard anything about it and did not seem very interested in what the presenter had so say and show. I was not surprised not because the feature is insignificant, but mostly because .NET developers are naturally not the best audience for database presentations, let alone for new BI functionality which is tightly coupled with Excel.

However, my eyes and ears were wide open for what the Technology Specialist had to say and he did alright – showed us how we can use Web Slices from SSRS 2008 R2 as a data source for PowerPivot, how we can publish reports through Excel Services, sorting and filtering 101 million rows in less than a second, etc. It was quite impressive.

After the presentation a senior developer approached me (since I am the BI go-to guy) and asked me, among other easy questions, the following: “Who will be using all this?” and “How will users upload 101 million rows spreadsheet to SharePoint?”

Since I had no straight answer for him, I thought it might be a good idea to ask the Microsoft national technology specialist. The replies (not literal quotes) we got were:

“How will users upload 101 million rows spreadsheet to SharePoint?”

Because PowerPivot uses column compression, 101 million rows actually take between 10 and 15 Mb, so uploading them over a network connection to SharePoint is not a problem.

“Who will be using all this?”

I understand that the potential use scenarios for PowerPivot are quite limited, but we (Microsoft) believe that there will be some cases where power-users will need and want this sort of functionality.

Fair enough. However, both of the answers were not convincing. The problems with them are:

1. What happens when we have a large number (say 1-2 million) dimension values, and a large number of facts as a source for PowerPivot? Obviously, column compression will not work that well in this case, which could be quite common.

2. If PowerPivot is mainly for power-users, how powerful do they need to be to take advantage of it? It seems like only a tiny subset of power-users with some expert Excel knowledge will be able to operate this new Excel functionality. I will be quite surprised to see users managing live feeds, analysis services sources, DAX, etc.

3. If BI developers first have to build a PowerPivot environment for the end-users, this does not seem like self-service BI at all, and I would be reluctant to do that if I can achieve the same and more though Analysis Services.

What I liked and I think would be quite useful are the numerous improvements to Excel 2010 as a data analysis and report generation tool. The product seems quite improved and I believe that it will get adopted quickly by heavy Excel users. However, I am somewhat perplexed by the buzz around self-service BI and PowerPivot in particular. Especially since we (BI developers) still need to get some extra functionality implemented to make our and our users’ everyday experience smoother and easier.

Please feel free to let me know if you have better answers to my questions. For the time being I consider myself instead of an opponent, an unconvinced and cautious observer of the recent developments in the BI space, and from this point of view self-service BI seems a bit overhyped. Hopefully it’s me – not the product, what needs refinement.

PowerPivot PowerPivot, self-service BI