16.14.6. Implementation
Triples entailed by subclass or subproperty statements in an inference context are not physically stored. Such triples are added to the result set by the query run time as needed. Also queries involving subclass or subproperty rules are not rewritten into unions of all the possible triple patterns that might imply the pattern that is requested. Instead, the SQL compiler adds special nodes that iterate over subclasses or subproperties at run time. The cost model also takes subclasses and subproperties into account when determining the approximate cardinality of triple patterns.
In essence, Virtuoso's support for subclasses and subproperties is backward chaining, i.e. it does not materialize all implied triples but rather looks for the basic facts implying these triples at query evaluation time.