Hierarchical Navigable Small World

Also known as: HNSW

HNSW is a graph-based approximate nearest neighbor index that organizes vectors into a layered navigable small-world graph. A search starts at the top layer — which has few, widely spaced nodes — and greedily descends through layers to progressively closer neighbors in the base layer. HNSW delivers excellent recall-vs-latency tradeoffs and supports incremental inserts without full index rebuilds, making it the default algorithm in most production vector databases including Pinecone, Weaviate, and Qdrant.