Semantic search and RAG rest on one deceptively simple operation: find the nearest vectors to a query. At a few thousand rows that's a boring loop; at a hundred million it's the whole ballgame. Here's what a vector index actually does inside — why brute force falls over, why HNSW's layered graph wins, and which three knobs decide your recall, latency, and RAM bill.
A modest app somehow grew Postgres, Redis, RabbitMQ, Elasticsearch and a vector DB — five things to back up, secure and pay for. Most of that is now one Postgres. Here's the queue, vector, search and pub/sub SQL, and the honest signals for when to graduate.