Your vector index won't fit in RAM at a billion vectors. How do you choose between HNSW, IVF-PQ, and disk-based ANN?

At a billion vectors the index choice is a memory budget problem before it's a recall problem. Flat search is out, HNSW may not fit, and PQ trades recall for RAM. Here is the decision a staff engineer makes on the whiteboard.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.