FAISS

FAISS (Facebook AI Similarity Search) is a C++/Python library for fast approximate nearest-neighbor search over dense vectors. The de-facto baseline for vector indexing — supports flat (exact), HNSW, IVF, PQ, and combinations.

For local RAG, FAISS is what's under the hood of Chroma, LangChain's default vector store, and many in-process embeddings setups. Index choice matters: HNSW is fast and accurate but memory-heavy; IVF-PQ trades recall for 10× smaller indexes.

For corpora under ~1M chunks, flat FAISS (exact nearest neighbor) is fast enough on CPU and avoids approximate-recall surprises. Beyond that, HNSW is the standard pick.