Restrictions on Outer Join Definitions
The following restrictions apply to outer joins when used to define a join index.
FULL OUTER JOIN is not valid.
When you specify a LEFT or RIGHT outer join, the following rules apply:
The outer table joining column for each condition must be contained in either column_1_name or column_2_name.
The inner table of each join condition must have at least one non-nullable column
in either column_1_name or column_2_name.