How does OBIEE choose its fact source in the Business Model?

Question

Hi,
We are currently experiencing a serious issue in the rpd concerning aggregate tables, the situation is as follows:
We have 2 fact tables,
1. Fact-General and
2. Aggregate table, Fact_Agg
And a couple of dimensions:
1. d_Date
2. d_Time
We also have a couple of measures defined, both mapped to the 2 fact tables:
1. Ent (sum)
2. Uni Ent (Count Distinct of a numeric key field)
When we query for Date and Uni Ent for instance, we would expect OBIEE to query the Fact_Agg table instead of the Fact_General (because it is the more efficient), but it always goes to the latter unless we disable it. The principle however works well for the Ent measure which hits the appropriate table (aggregate when necessary) accordingly when queried.
Can you please advise as to whether we have wrongly implemented something or is it that aggregate tables do not properly handle Count Distinct measures (which we doubt)? If possible, could you outline the mechanism by which OBIEE selects which table to query - what factors does it take into account?
N.B : Please let me know if you need any other information.
Many thanks and Regards
N.

Answer 1

Hi Chino,
Thank you for your valuable information. However, what i do not understand is why for another measure defined as count(*), the principle works fine and it does hit the aggregate when appropriate, but for this particular measure (count distinct) it hits the general fact instaed of the aggr. table.
Does it mean that aggr tables cannot handle count distinct measures properly?
Many thanks for your response,
N.