Every RAG tutorial pulls the same move. It walks you through embeddings, chunking, retrieval, and then right at the moment of truth it says "now pick a vector database" and just... moves on. Like that's a coffee-order decision. Like you're choosing a font. It's not. The database you pick decides how fast your queries come back, how much you bleed in infrastructure costs, and how miserable your life gets the day traffic triples. And here's the thing that took me a while to figure out while resear