support for attr-only virtual schema; fixed issues with joinability of those cases
support for attr-only virtual schema; fixed issues with joinability of those cases

mitdbg/aurum-datadiscovery,mitdbg/aurum-datadiscovery,mitdbg/aurum-datadiscovery,mitdbg/aurum-datadiscovery,mitdbg/aurum-datadiscovery,mitdbg/aurum-datadiscovery