I dug into why we hit a score diff when using luceneutil to benchmark
At first I thought it was BS1/BS2 difference, but because of a bug in
the patch it was still using BS2 (but should be BS1) – Robert's last
patch fixes that.
But it's actually a diff in BS2 itself, whether you next or advance
through the docs.
It's because DisjunctionSumScorer, when summing the float scores for a
given doc that matches multiple sub-scorers, might sum in a different
order, when you had .nextDoc'd to that doc than when you had .advance'd
This in turn is because the PQ used by that scorer (ScorerDocQueue)
makes no effort to break ties. So, when the top N scorers are on the
same doc, the PQ doesn't care what order they are in.
Fixing ScorerDocQueue to break ties will likely be a non-trivial perf
hit, though, so I'm not sure whether we should do anything here...