Asked about Retrieve broadly, then rank precisely
Why use two retrieval stages?
An answer that works
Recall and precision want different things. The first stage is cheap and broad so the answer is definitely in the candidate set. The second is expensive and accurate and orders a short list. One stage has to compromise between them.
Reading an answer is not being able to give one. The section that teaches this is Retrieve broadly, then rank precisely, 6 hours of Reranking & hybrid search.
Learn this properlyAlso asked about Reranking & hybrid search
Why fuse search results by rank rather than score?What is a cross-encoder and why not use it for everything?Your RAG chatbot answers the first question well and follow-ups badly. Why, and what do you do?What is MRR and when would you use it rather than recall@k?When would you search with a hypothetical answer rather than the question, and what is the risk?How do you decide which query techniques to keep?