Implicit Fact Column - am I thinking about this the right way?

Question

My exploration of the Implicit Fact Column is below. Let me know if you think this is the intended usage.
When the BI Server sees a request where one Dimension is being constrained by another, it has to pick a fact table that contains references to these two dimensions. The "Implicit Fact Column", which is set in the Presentation Layer of the RPD is used to guide the BI Server towards one or more Fact tables that can be used to satisfy the request. In the absence of the Implicit Fact Column, I noticed that the BI Server was choosing random fact tables so I went exploring how this feature worked. I set the Implicit Fact column in my Presentation layer and then went looking to see how the queries were generated.
For my example, my two dimensions are Program and Channel. I want to see the list of Channels available for a Program. Without the Implicit Fact Column, the queries are of the form:
select distinct T289.CHANNEL_NAME as c1,
T36001.PROGRAM_NAME as c2
from
DIM_PROGRAMS T36001,
DIM_CHANNELS T289,
SomeRandomFactTable T41210
where ( T289.DIM_CHANNEL_KEY = T41210.DIM_CHANNEL_KEY and T36001.DIM_PROGRAM_KEY = T41210.DIM_PROGRAM_KEY )
order by c1, c2
The nice part about this is that there is only one set of Program / Channel combinations returned. The downside is that the Fact table that is chosen to resolve the combinations can be any Fact table under the purview of the BI Server. It doesn't even have to be in the same subject area where the Program and Channel were chosen. When I set the Implicit Fact Column, the queries are of the form:
select distinct T289.CHANNEL_NAME as c1,
T36001.PROGRAM_NAME as c2,
T35832.DIM_SITE_KEY /* this is the implicit fact column */
from
DIM_PROGRAMS T36001,
DIM_CHANNELS T289,
AFactTableMappedToFactColumn T35832
where ( T289.DIM_CHANNEL_KEY = T35832.DIM_CHANNEL_KEY and T35832.DIM_PROGRAM_KEY = T36001.DIM_PROGRAM_KEY )
order by c1, c2
I was happy that I could predict what Fact table was being used, but now the results are wrong as I get a duplicate set of records for every DIM_SITE_KEY / Channel / Program combination. I decided to play a little Algebra trick. Knowing that the DIM_SITE_KEY was a numeric value, I defined the Implicit fact column as DIM_SITE_KEY / DIM_SITE_KEY which will always result in a value of 1. Now I get queries that look like:
select distinct T289.CHANNEL_NAME as c1,
T36001.PROGRAM_NAME as c2,
T35832.DIM_SITE_KEY / nullif( T35832.DIM_SITE_KEY, 0) as c3
from
DIM_PROGRAMS T36001,
DIM_CHANNELS T289,
AFactTableMappedToFactColumn T35832
where ( T289.DIM_CHANNEL_KEY = T35832.DIM_CHANNEL_KEY and T35832.DIM_PROGRAM_KEY = T36001.DIM_PROGRAM_KEY )
order by c1, c2
Since DIM_SITE_KEY / DIM_SITE_KEY is always equal to 1, I only get one set of Program / Channel combinations. I get the added bonus of knowing which set of Fact tables are going to be used to satisfy the request.

Answer 1

Perfect Analysis, But one important note is when using implicit fact table, Always select measure( Which has some aggregation) as a implicit fact column. So that will allows the query will eliminate duplicate rows by doing a group by operation.
Other note is make sure performance is good. When we don't have a good model for fact tables, when the tables contains large set of data, make sure performance is good by doing indexes or caching to load the prompts.
- Madan