Interview Bank

RAG in 14 Days: From Retrieval to Trustworthy Answers

D10 Query-Side Optimization: Rewriting, Hypothetical Document Embeddings, Multi-Query, Step-Back Prompting, and Intent Routing

• How do you handle coreference in multi-turn RAG, and what is the classic failure when you skip it?多轮对话里怎么处理指代？不做指代消解最典型的翻车场景是什么？

  Common in ChinaCommon overseasBasic#coreference#multi-turn#query-rewriting

  How to reason about it · think before answering

  1. This is a warm-up question, but there is still a gap between answers. Saying "just concatenate the history into the query" invites a follow-up about growing histories that most candidates cannot handle.
  2. State the mechanism: insert a short rewrite call before retrieval that takes the last few turns plus the current question and returns one retrieval-ready line. Set temperature to 0 so the same input always yields the same query, and forbid the model from answering the question in the prompt.
  3. Explain why concatenation is worse: history grows without bound, filler words dilute inverse document frequency, and the previous answer leaks in — you end up retrieving an answer with an answer. The rewriter emits one sentence, not a transcript.
  4. Make the failure concrete. Turn one: "who must sign off on this operation?" Answer: "the platform team lead." Turn two: "what is that person's name?" Retrieved unresolved, not a single candidate clears the admission gate and the system refuses — even though the corpus contains the answer. The failure is not a wrong answer, it is a false "not found" right after the user's own question.
  5. Add the ordering trap: rewrite before intent routing. A pronoun is a classic multi-hop signal, so an unresolved query gets routed to the expensive path for nothing; after rewriting it is an ordinary single-hop question. Multi-query and step-back must also sit downstream of the rewrite, or one unresolved pronoun becomes three.
  6. Expect "how do you decide when to rewrite?" Trigger on short queries, pronouns and elliptical follow-ups; skip on a clearly new topic. The check is nearly free and removes most of the calls.

  分析过程 · 先想清楚再作答

  1. 这是一道送分题，但送分题也有高下之分：只说「把历史拼进查询里」的答案，会被追问一句「历史越拼越长怎么办」就卡住。
  2. 先把做法说清：在检索之前加一次很短的改写调用，输入是最近几轮对话加本轮问题，输出是一行可以直接检索的检索式；温度设 0 保证同一句话每次改成同一个结果，并在提示词里明确禁止模型顺手回答问题。
  3. 为什么不是「把历史整个拼进查询」：历史越拼越长，噪声词把逆文档频率摊薄，检索反而更差；而且历史里包含上一轮的答案，等于拿答案去检索答案。改写的产出是一句话，不是一段历史。
  4. 最典型的翻车场景要举实例：上一轮问「这个操作必须由谁审批」，答「必须由某某组组长审批」；这一轮问「这个人叫什么名字」。不消解直接检索这七个字，实测是**一条候选都过不了门槛，系统只能拒答**。注意失败方式不是答错，是「明明语料里有答案却说找不到」，用户体验是崩塌式的。
  5. 补一条顺序上的坑：改写必须在意图路由**之前**。「这个人」是典型的多跳信号词，路由看到它会判成多跳、白跑一轮；改写之后它只是个普通单跳问题。同理，多路查询、后退提问也都要建立在改写后的那句话上，否则错误被放大好几倍。
  6. 可预期的追问是「怎么知道要不要改写」。答：短问题、含指代词、含省略（「那审计日志呢」）时才触发，纯新话题跳过——这一步很便宜，但能省掉一大半调用。

  Key points

  • Add a short rewrite call before retrieval: last few turns plus current question in, one retrieval line out, temperature 0, answering explicitly forbidden.
  • Do not splice the whole history into the query — it grows unbounded, dilutes IDF, and leaks the previous answer into the search.
  • Classic failure: an unresolved pronoun means no candidate clears the gate, so the system refuses a question the corpus can answer.
  • That false "not found" hurts more than a wrong answer, since the user just asked about the same thing.
  • Order matters: rewrite first, then route; multi-query and step-back both build on the rewritten query.

  答题要点

  • 在检索前加一次短改写调用，输入最近几轮加本轮问题，输出一行检索式，温度 0，禁止模型回答问题。
  • 不要把历史整段拼进查询：越拼越长、噪声稀释逆文档频率，还会拿上一轮的答案去检索。
  • 典型翻车：上一轮的「这个人 / 他 / 那个」不消解，检索一条都过不了门槛，系统在有答案的情况下拒答。
  • 失败方式是「假的查不到」，比答错更伤体验，因为用户刚刚才问过同一件事。
  • 顺序：先改写、再路由，多路查询与后退提问都建立在改写后的查询上。