Outer Join Processing

The optimizer processes outer joins by forcing all outer joins into left outer joins (right outer joins are converted into left outer joins by simply reversing the order). It then will disable all access paths that require the right hand table to be accessed before the left hand table. If there is no access path (that is, through an index or declared foreign key) from the left hand table to the right hand table, the optimizer will reject the query with an error indicating that no outer join access path is available.