16.5.10. Join Optimization -- Declaring IRI
Additional problem appears when the equality is between two IRIs
of two different IRI classes. Even if both of them are bijections,
the compiler does not know if these IRI classes behave identically
on the intersection of their domains. To let the optimizer know
this fact, one IRI class can be explicitly declared as a subclass
make oplsioc:user_iri subclass of oplsioc:grantee_iri .
make oplsioc:group_iri subclass of oplsioc:grantee_iri .
The SPARQL compiler can not check the validity of a subclass
declaration. The developer should carefully test functions to
ensure that transformations are really subclasses, as well as to
ensure that functions of an IRI class declarations are really
inverse to each other.
When declaring that a table's primary key is converted into a
IRI according to one IRI class, one usually declares that all
foreign keys referring to this class also get converted into an IRI
as per this same class, or subclass of same class.
Subclasses can be declared for literal classes as well as for
IRI classes, but this case is rare. The reason is that most of
literals are made by identity literal classes that are disjoint to
each other even if values may be equal in SQL sense, such as
"2" of type xsd:integer and "2.0" of type xsd:double .