parent document retrieval

Parent document retrieval indexes small chunks for precision but returns larger parent documents for context. **Problem**: Small chunks retrieve precisely but lack context; large chunks have context but imprecise retrieval. **Solution**: Index small chunks (sentences/paragraphs), link each to parent (page/section), retrieve by small chunk but return parent to LLM. **Implementation**: Store mapping: small_chunk_id → parent_chunk_id. At retrieval: find relevant small chunks → look up parents → return deduplicated parents. **Chunk hierarchy**: Sentence (retrieval unit) → paragraph → section → document. Can have multiple levels. **Trade-offs**: Returns more text (larger context windows needed), may include some irrelevant content from parent. **LangChain support**: ParentDocumentRetriever built-in. **Variations**: Retrieve then expand (fetch N surrounding chunks), multi-granularity (retrieve at multiple levels). **Tuning**: Balance child chunk size (precision) vs parent size (context). **When to use**: When context matters (narratives, technical explanations), when relationships between sentences are important. Widely adopted pattern in production RAG.