I really think that the time dimension is the most complex dimension in 90% of the DWHs. The complexity is in two places: In the DWH design and also in Analysis Services (or any other BI tool).
First of all - why we didn't take the already-made Server Time Dimension which exists in SSAS 2005? For two reasons: The first is that the
Project Real guys do not recommend using it (you can find their SSAS article
here). The second is that we wanted to have some features that are not available in the server time dimension, such as Hebrew date. In a matter of fact, even if we didn't have such feature we still would build the time dimension ourselves because it's giving you much more control over the dimension. For example, you can always add some new attributes which Microsoft developers didn't think about.
I started myself to build the time dimension in excel. I figured out that this mission is little more complex that I thought it would. Most of the functions I wrote were simple, but there were some complicated ones. So here are some tips for you if you want to build your time dimension using Excel:
- If you want to week number for every date, do not write the function yourself... Excel has function called weeknum. If you don't have it just add the function toolbox which has it (I can't recall its name right now. check in excel help).
- If you want to have records for every level in your hierarchy (not only for days), put every level in its own excel file (not excel tab). It will help you later when you will transfer it to your DB.
- Check yourself. Pick randomly some dates and check that all of its record has correct data.
After building the excel files I needed to transfer it to my Oracle server. I used SSIS because I didn't want to wait for my DBA to copy these files into the Informatica server (it can't use my the local files, it has to be in its server. SSIS can use local files). This also was a little tricky. First of all, close excel when running the SSIS packages, otherwise it will fail. Second, when moving the non-leaf levels, go into the columns section in the destination box and erase the irrelevant columns. It will reduce the chance for errors. Finally, click on the source box and click on "Show advanced editor". Enter the source's output columns options and define properly the columns' data types. This also will reduce the chance for errors.
I had a little bit of an argument with my DBA about how should the time dimension be. I think that the time dimension does not have to be processes at all. My time dimension is from 1960 until 2020, so no daily ETL is required. She says that all the logic has to be in Informatica so I need to develop a mapping for this dimension. I think that we both are right and that's because that in ideal world she is right. In every developers team, all the BL has to be in one place. But we don't have much time (the deadline is very close) so I won't spend the time building more mapping in Informatica when I have the time dimension already made in excel.
Maybe someday I will have the time to do this. Maybe not.