So I want to ask for advice on 2 related topics:

1. If I have a corpus of many documents embedded in a vector store, how can I dynamically select (by metadata, for example) a subset of them and only perform retrieval on that subset for answer generation.

2. I want LLaMa to be able to say I DO NOT KNOW if the context it retrieved cannot answer the question. This behavior is not stable yet from what I have seen.

Thank you so much!