What is the HNSW index in ChromaDB and what parameters can you tune?

ChromaDB uses HNSW (Hierarchical Navigable Small World) as its Approximate Nearest Neighbour (ANN) index. HNSW builds a layered graph structure where each node connects to its closest neighbours — queries traverse this graph efficiently to find approximate nearest neighbours in O(log n) time instead of exhaustive O(n) linear scan.

import chromadb

client = chromadb.Client()

# HNSW parameters are set as metadata at collection creation
collection = client.create_collection(
    name="tuned_collection",
    metadata={
        "hnsw:space":           "cosine",   # distance metric
        "hnsw:construction_ef": 200,         # default 100
        # Controls quality of index during construction.
        # Higher = better recall, slower inserts.

        "hnsw:search_ef":       100,         # default 10
        # Controls quality of search at query time.
        # Higher = better recall, slower queries.

        "hnsw:M":               16,          # default 16
        # Number of bi-directional links per node.
        # Higher = better recall + more memory + slower inserts.
        # Typical range: 4-64.
    },
)

# Note: HNSW parameters cannot be changed after collection creation
# You would need to recreate the collection and re-insert data

collection.add(
    documents=[f"Document number {i}" for i in range(10000)],
    ids=[str(i) for i in range(10000)],
)

For most RAG use cases, the defaults work well for collections under ~100K documents. For large collections or when recall matters, increase hnsw:search_ef to 50–200 and set hnsw:construction_ef to at least 200 when building the index.