Exact Nearest Neighbor

Also known as: ENN, flat search, brute-force search

Exact nearest neighbor search computes the distance between the query vector and every vector in the index, returning the true closest neighbors with 100% recall. It is computationally infeasible for large indexes — latency grows linearly with corpus size — but is useful for small datasets, offline evaluation, and as the recall benchmark against which ANN algorithms are measured. FAISS's IndexFlatL2 is a widely used flat-index implementation for exact search.