1. Performance degradation: we have quite heavy custom implementation of score(). So when we started using DrillSideways, this call became top-1 in a profiler snapshot (top-3 with default scoring). We tried doUnionScoring and doDrillDownAdvanceScoring, but no luck:
doUnionScoring scores all baseQuery docIds
doDrillDownAdvanceScoring avoids some redundant docIds scorings, considering symmetric difference of top two iterator's docIds, but still scores some docIds, that will be filtered out by 3rd, 4th, ... dimension iterators
doQueryFirstScoring scores near-miss docIds
Best way is to score only true hits (where baseQuery and all N drill-down iterators match). So we suggest a small modification of doQueryFirstScoring.
2. Speaking of doQueryFirstScoring, it doesn't look like we need to calculate a score for near-miss hit, because it won't be used anywhere.
FacetsCollectorManager creates FacetsCollector with default constructor
so FacetCollector has false for keepScores
and collectScore is not being used