Design Tips on OBIEE BMM Layer

The following is a list of tips to use when designing the Business Model and Mapping layer:

1. Create the business model with one-to-many complex joins between logical dimension tables and the fact tables wherever possible. The business model should ideally resemble a simple star schema in which each fact table is linked directly to its dimensions.
NOTE: If you use the snowflake model, you might have more flexibility (for example, the ability to use outer joins) but it may create more columns in the presentation layer. However, it is recommended that you minimize the use of snowflake schemas.

2. Outer joins should be avoided in the reporting SQL. They can be eliminated in the ETL via a variety of techniques, but by doing so, not only can an additional table be removed from the report SQL, but the performance will also improve.

3. Combine all similar dimensional attributes into one logical dimension table. Where needed, include data from other dimension tables into the main dimension source using aliases in the Physical layer tables. This should occur during the ETL process for optimal performance.

4. Every logical dimension table should have a dimensional hierarchy associated with it. Make sure that all appropriate fact sources link to the proper level in the hierarchy using aggregation content. You set up aggregation content in the Content tab of the Logical Table Source properties window.

5. Aggregate sources should be created as a separate Source in a logical table. For fact aggregates,use the Content tab of the Logical Table Source properties window to assign the correct logical level to each dimension.

6. It is recommended that you use table aliases frequently in the Business Model layer to eliminate extraneous joins, including the following:

7. Eliminate all physical joins that cross dimensions (inter-dimensional circular joins) by using aliases.

8. Eliminate all circular joins (intra-dimensional circular joins) in a logical table source in the Physical Model by creating physical table aliases.

9. Renaming an element in the Business Model and Mapping layer will automatically create an alias.

10. Outer joins in logical table sources are always included in a query, even if the table source is not used. If possible, create one logical table source without the outer join and another with the outer join. Order the logical table source with the outer join after the non-outer join so that it will be used only when necessary.

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