Skip Level Dimensions

Question

Dear all
We have a client with an interesting problem. I was wondering if the group could offer any feedback on this design question, where we think it involves a concept called 'skip levels'. The client is using Oracle 9.0.2 with their data mart cube held as ROLAP. Our first thoughts are that what they want to do is better served by using a true OLAP tool such as Oracle Express, or Oracle 9i OLAP, which deals with skip level dimensions easier. Anyway, here's the problem :
"For our warehouse project, we need some dimensions (initially: account, company) that use a skip level hierarchy. This means that the lowest level dimensions are of the same level (G/L account or company) and they need to roll up in a structure of varying levels, before they come together again in a highest level.
Ideally, we would like to use a data structure that can be used transparently by the user and allows for rollups of amounts through the dimension. We would like to be able to populate the dimension through OWB and to query the data using both Crystal reports v9 and BI beans and possibly other tools that recognize the Oracle data warehouse concept.
The two solutions we are aware of are:
1.     Insert Dummy levels:
This is not a nice solution but is will work and it is simple.
2.     Insert a helper table that contains the higher level account with all the children (as per Ralph Kimball)
This seems a cumbersome solution but I am sure it would work also. The question is how the Oracle's BI beans and the CWM2 metadata would recognize this and how we can make this transparent to the user.
I am sure there are other solutions to the problem. It seems like the problem would be solved by the next release of Oracle (9.0.3?), which allows us to deal with skip level hierarchies through CWM2 (which I believe this is). The solution we choose should also allow us to easily upgrade to the new skip level hierarchy, which I would expect to be easier to use then the above."
The main thing to bear in mind here is that they want to implement a ROLAP solution, using Oracle 9i tables, dimensions and heirarchies. It also has to work with Oracle Warehouse Builder, take advantage of the CWM2 metadata (if possible) and work with the BI Beans.
Any thoughts on this? Does anyone know how Oracle are going to cater for skip-level dimensions with the next release? Is this through the improved support for the CWM2 metadata standard for ROLAP cubes, or is the customer best of going to the true OLAP 'Analytic Workspace'? If we ignore a MOLAP solution, what's the best way of dealing with this, in such a manner that it's transparent to query tools, and isn't too much of a 'hack'?
Many thanks in advance,
Mark Rittman
Mark Rittman
Consulting Manager, Plus Consultancy
[email protected]
[email protected]--------------

Answer 1

As part of my search i've uncovered an article by Ralph Kimball on using 'helper tables' to deal with ragged hierarchies.
http://www.dbmsmag.com/9809d05.html
This advocates using a table between the fact table and the dimension. Quoting from Mr. Kimball;
"You can solve this modeling problem by inserting a helper table between the dimension table and the fact table, as shown in Figure 3 (http://www.dbmsmag.com/9809d05.html#fig3). Amazingly enough, you don't have to make any changes to either the dimension table or the fact table; you just rip the join apart and insert the helper table.
The helper table contains one record for each separate path from each node in the organization tree to itself and to every node below it. There are, then, more records in the helper table than there are nodes in the tree. In Figure 3 we need a total of 43 records in the helper table. See if you can work this out.
Each record in the helper table contains the fields:
- Parent Customer Key
- Subsidiary Customer Key
- Depth From Parent
- Lowest Flag
- Topmost Flag.
If you are descending the tree from certain selected parents to various subsidiaries, you join the dimension table to the helper table and the helper table to the fact table with the joins as shown in Figure 3. The Depth From Parent field counts how many levels the subsidiary is below the parent. The Lowest Flag field is True only if the subsidiary has no further nodes beneath it. The Topmost Flag field is True only if the parent has no further nodes above it.
The beauty of this design is that you can place any normal dimensional constraint against the Customer dimension table and the helper table will cause all the fact table records for the directly constrained customers plus all their subsidiaries to be correctly summarized. In other words, you can use your standard relational databases and your standard query tools to analyze the hierarchical structure."
My question is - has anyone tried to implement this with OWB, or with the Oracle ROLAP data warehouse in general, reporting through Discoverer or BI Beans? Any opinions?
thanks
Mark