Using Outer Joins to Define Simple Join Indexes
Because join indexes generated from inner joins do not preserve unmatched rows, you should consider using outer joins to define simple join indexes, noting the following restrictions.
Defining join indexes on outer joins enables them to satisfy queries with fewer join conditions than those used to generate the index. See “Defining a Simple Join Index on a Binary Join Result” on page 542 and “Using Outer Joins to Define Join Indexes” on page 559 for a demonstration of this property.
Also see “Restriction on Coverage by Join Indexes When a Join Index Definition References More Tables Than a Query” on page 577 and related examples.