VCS: improve proxy-based search performance

VCS searches involving proxies need to do a
LEFT JOIN proxies ON proxies.id = _nxhier.id
JOIN hierarchy ON (hierarchy.id = _nxhier.id OR hierarchy.id = proxies.targetid)
and the OR part is very costly.

Find a way to change this, either by using a UNION ALL of two queries (one one normal docs, the other on proxies), or something else.