面试题库

14 天 RAG：从检索到可信回答

D11 高级索引：父子文档、摘要索引、上下文检索，以及树状聚合与图检索的取舍

• 什么样的问题必须上图检索？给一个该上的具体例子和一个不该上的例子。What kind of question actually requires graph retrieval? Give one concrete case where it is justified and one where it is not.

  国内高频海外高频深入#graph-rag#multi-hop#cost

  分析过程 · 先想清楚再作答

  1. 这题在考你会不会为了用而用。只要答案里出现『多跳问题就要上图检索』，面试官基本就知道你没落地过——多跳只是必要条件，远不是充分条件。
  2. 判据要落在一个可观察的现象上：**答案的第二篇文档和查询之间，有没有字面或语义上的重合**。有重合，普通的混合检索就能捞到它，多跳是假的；完全没有重合，只能靠一条关系边走过去，这才是图检索的领地。
  3. 该上的例子：问『生产库主备切换必须谁书面审批、这个人叫什么』。一篇写着须平台组组长审批，另一篇写着平台组组长是某人。第二篇跟查询一个词都不重合，我们在五种索引结构下测了一遍，它在 20 条候选池里一次都没出现过——换切法、加块头、父子回填全都无效。
  4. 不该上的例子：问『扩容要走哪个流程、最晚提前几个工作日提单』。同样跨两篇文档，但两篇都跟查询有明显字面重合，混合检索把它们分别排在第 2 名，一次检索就凑齐了。为它建图是拿几倍成本买一个已经解决的问题。
  5. 然后说代价，这一段决定了你像不像做过：建图不止一次抽取调用，实体要消歧、关系要去重、文档更新时受影响的子图要重算，还要多维护一套图存储和一套更新链路。
  6. 可预期的追问是『不上图检索还有什么办法』。答案是把多跳交给 Agentic 检索：让模型先查出中间实体，再拿这个实体发起第二次检索。它的一次性成本几乎为零，代价换成了每次查询的延迟与调用次数——先试这条，试不通再考虑建图。

  How to reason about it · think before answering

  1. This one tests whether you reach for tools you don't need. If the answer is 'multi-hop questions need a graph', the interviewer knows you haven't shipped one — multi-hop is necessary, nowhere near sufficient.
  2. Anchor the criterion on something observable: does the second required document share any lexical or semantic overlap with the query? If it does, ordinary hybrid retrieval will surface it and the hop is illusory. If it shares nothing, only a relation edge gets you there — that is graph territory.
  3. Justified case: 'who must sign off on a production failover, and what is that person's name?' One doc says the platform lead must approve; another says who the platform lead is. The second shares not one term with the query. Across all five index structures we tested, it never once appeared in a 20-item candidate pool — rechunking, headers and parent backfill all failed.
  4. Unjustified case: 'which process covers a capacity change, and how many working days ahead must the ticket be filed?' Also two documents, but both overlap the query lexically; hybrid retrieval ranked them second each, and one pass collected both. Building a graph for this buys a solved problem at several times the cost.
  5. Then state the cost, which is what makes the answer sound operational: graph building is not one extraction call. Entities need disambiguation, relations need dedup, updates force recomputing affected subgraphs, and you now run a graph store and its update pipeline.
  6. Expect 'what else could you do instead'. Hand multi-hop to agentic retrieval: let the model retrieve the intermediate entity first, then issue a second query with it. Near-zero build cost, paid back in latency and call count per query. Try that before you build a graph.

  答题要点

  • 判据不是『是不是多跳』，而是『第二篇文档跟查询有没有字面或语义重合』——没有重合才轮得到图检索。
  • 该上：审批人那类问题，中间实体是唯一的桥，第二篇文档在候选池里一次都不出现。
  • 不该上：两篇都跟查询有重合的多跳题，混合检索一次就能凑齐。
  • 建图的真实成本是实体消歧、关系去重、增量重算和一套额外的图存储，不是一次抽取调用。
  • 先试 Agentic 检索的两次查询，走不通再考虑建图。

  Key points

  • The test is not 'is it multi-hop' but 'does the second document overlap the query at all' — only zero overlap earns a graph.
  • Justified: the approver question, where an intermediate entity is the only bridge and the second doc never enters the candidate pool.
  • Not justified: a multi-hop question whose documents both overlap the query — hybrid retrieval collects them in one pass.
  • Real graph cost is entity disambiguation, relation dedup, incremental subgraph recomputation and a whole extra store — not a single extraction call.
  • Try two-pass agentic retrieval first; build the graph only when that fails.