Filtered Vector Search: What ACORN Fixes, and What Fixes ACORN

Filtered vector search breaks when metadata filters turn a healthy nearest-neighbor graph into scattered islands. HNSW’s m parameter controls how many links each point gets; at Qdrant’s default m=16 , the collection we benchmark below averaged about 21 links per node on layer 0. Filter out 96% of the points and fewer than one link per node survives on average, so traversal gets stranded before it reaches the true nearest matches.