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 question before it's a recall question. Flat search is off the table, HNSW may not fit, and PQ swaps recall for RAM. Here is the call a staff engineer makes on the whiteboard.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
At a billion vectors the index choice is a memory budget question before it's a recall question. Flat search is off the table, HNSW may not fit, and PQ swaps recall for RAM. Here is the call a staff engineer makes on the whiteboard.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
