Constant-object DELETE over many subjects reads one slot per subject #2060

With #2008 a retraction names a stored fact, so a constant-object DELETE (DELETE { ?s ex:p "v" } WHERE { ?s ex:q ?x }, DELETE DATA of many subjects) reads each (graph, subject, predicate) slot to find the fact. On an indexed ledger that costs 1.09–1.13× main on #2008's bench, where main committed phantom retractions without reading.

One POST read per (predicate, object) pair would serve all of a pair's subjects. It needs a novelty read by pair beside today's per-slot one, and a bound for popular pairs, where one POST read would scan every subject holding the value while the DELETE names a few.

Found while reviewing #2008 (listed in its follow-ups).