ann

Everyone reaches for a bigger machine. The two mechanisms that actually cause this both scale with corpus size, both hit the tail before the median, and one of them is the permission filter your security review insisted on.

The document count matches, nothing threw, and a specific set of documents is now unreachable by any query. That combination narrows the cause fast, and the two survivors are an approximate-search parameter that a rebuild reset and a metadata filter applied in the wrong order.

Brute force is O(N*d) per query and dies at ten million vectors. The answer is approximate nearest neighbor: trade a little recall for orders of magnitude less work, and know exactly which knob buys what.

10 billion vectors is where raw float storage (30TB) stops being an option. The answer does the HNSW vs IVF-PQ RAM math cold and shows why PQ is the only thing that fits in memory.