[Retrieval](https://js.langchain.com/docs/concepts/retrieval/): LLM should read this page when (building question answering systems), (working with various data sources), (implementing natural language interfaces) (Overview of retrieval systems for unstructured data and structured data. Details on query analysis using models to construct or re-write queries. Information on different retrieval systems like vector indexes, databases, and LangChain's unified retriever interface.)

