Interview Bank

RAG in 14 Days: From Retrieval to Trustworthy Answers

D4 Chunking Strategies: Five Approaches — Fixed, Recursive, Structure-Based, Parent-Child, and Semantic — and Choosing by Evaluation, Not Intuition

• What overlap ratio would you use, and what concretely goes wrong when the overlap is too large?重叠区设成块长的百分之多少合适？重叠过大会带来什么具体问题？

  Common in ChinaCommon overseasBasic#chunking#overlap

  How to reason about it · think before answering

  1. This is a giveaway question, but the marks are in the second half, not the percentage. Stopping at 'usually ten to twenty percent' reads like someone who has never run it.
  2. Say what overlap is patching: fixed-length splitting cuts sentences in half, and overlap guarantees the broken sentence survives intact in at least one of the two neighbours. It is a patch for careless splitting, not an optimisation of its own.
  3. That yields the first conclusion: with structural or recursive splitting the boundaries already land on semantic positions, so the need for overlap drops sharply and can legitimately be zero. The ratio question is meaningless without naming the strategy.
  4. Give three concrete costs. Storage and tokens: at 400-character chunks, moving overlap from 0 to 80 grows total index tokens by roughly fifteen percent, which is storage cost in the vector store and comparison work at query time.
  5. Retrieval redundancy: the more neighbours overlap, the more likely the top results are three versions of the same passage. You think you handed the model three pieces of evidence; you handed it one, three times. Nothing fixes this before reranking.
  6. Citation resolution: when a sentence lives in two chunks, which one does the model cite. Expect the follow-up on deduplication: merge at the result layer using a content fingerprint or longest common substring, not by tweaking the chunker.

  分析过程 · 先想清楚再作答

  1. 这是一道送分题，但送分点不在那个百分比上，而在后半句。只答「一般一到两成」就停住的人，面试官会认为他没跑过。
  2. 先说清重叠在补救什么：固定长度切法会把句子从中间切开，重叠让被切开的那句话至少在相邻两块之一里是完整的。它是给「乱切」打的补丁，不是一个独立的优化。
  3. 由此推出第一个结论：如果你用的是按结构切或递归切，边界本来就落在语义位置上，重叠的必要性会大幅下降，甚至可以是零。**重叠比例这个问题的前提是切法**，脱开切法谈比例就是背数字。
  4. 过大的代价要说三笔，越具体越好。存储与 token：块长 400、重叠从 0 加到 80，索引 token 会涨一成半左右，这笔钱在向量库是存储费、在检索时是比对量。
  5. 检索冗余：相邻块越像，前几名越可能是同一段话的三个版本，你以为给了模型三条证据，其实是一条说了三遍。这一条在重排之前基本无解。
  6. 引用定位：同一句话出现在两个块里，模型标出处该标哪一个，这会直接变成引用校验环节要处理的边界情况。可预期的追问就是「那你怎么去重」，答按内容指纹或最长公共子串在结果层合并，而不是在切块层想办法。

  Key points

  • Ten to twenty percent of chunk length is the working range, but that number assumes fixed-length splitting.
  • With structural or recursive splitting the boundaries are already semantic, so overlap can be small or zero.
  • Cost one: index tokens and storage grow noticeably; at 400-character chunks, an 80-character overlap adds roughly fifteen percent.
  • Cost two: neighbouring chunks become near-duplicates, so the top results are several versions of one passage and the evidence diversity is illusory.
  • Cost three: a sentence spanning two chunks complicates citation attribution and forces result-level deduplication.

  答题要点

  • 经验区间是块长的一到两成，但这个数字的前提是你用的是固定长度切法。
  • 按结构或递归切时边界本来就在语义位置上，重叠可以很小甚至为零。
  • 过大代价一：索引 token 与存储明显上涨，块长 400 时重叠加到 80 大约涨一成半。
  • 过大代价二：相邻块高度相似，检索前几名变成同一段话的多个版本，证据多样性是假的。
  • 过大代价三：同一句话跨块出现，引用标注和去重都要额外处理。