There were some special cases that were handled individually:
- select * { <s> ?p ?o } was handled through querying the rdf:types of <s>,
  and performing an UNION of all related tables. This doesn't play along
  with parameterized variables where the subject might be a variable assigned
  later through the TrackerSparqlStatement.
- select * { ?s ?p <o> } used similar tricks (querying <o> this time). It had
  all those drawbacks, plus it only worked for properties with rdfs:Resource
  range.
- select * { ?s ?p ?o . ?p rdfs:domain <dom> } was handled specially.
- select * { ?s ?p ?o } is unsupported
- select * { <s> ?p <o> } was silently unhandled

All those combinations are transparently handled with the tracker_triples
virtual table now.