What is parent document retrieval and when is it useful?

Parent document retrieval stores small chunks for precise embedding-based retrieval but returns the larger parent document or section to the language model after matches are found. This solves the trade-off between precision in retrieval, which favors small chunks, and sufficient context in generation, which favors large passages. It is standard in LlamaIndex and LangChain for document-heavy RAG on reports, textbooks, and legal documents where answers reference surrounding material that small chunks lose.