OBIEE: What is a Driving Table and when is it preferred ?

A driving table is a feature available in OBIEE using which one can improve performance of cross database joins. However this feature is very limited and can be counter productive. 

Here is how Oracle defines a Driving Table,

Driving tables are for use in optimizing the manner in which the Oracle BI Server processes cross-database joins when one table is very small and the other table is very large. Specifying driving tables leads to query optimization only when the number of rows being selected from the driving table is much smaller than the number of rows in the table to which it is being joined. 

When you specify a driving table, the Oracle BI Server will use it if the query plan determines that its use will optimize query processing. The small table (the driving table) is scanned, and parameterized queries are issued to the large table to select matching rows

This feature is available in the BMM(Business Model and Mapping Layer) of repository where you define a logical join between two tables, Here you will be able to see an option called Driving Table where you can select one of the two tables in the join. Make sure you are selecting the table with the less number of rows, usually preferred when data in one of the tables is very less ( <1000) and the other table is very large. 

Driving Table feature in Logical Join of OBIEE RPD.

Reference: Driving Table - Oracle