What is the difference between reindexing a vectorstore database after adding a new document and not reindexing? For example, if I initially have N documents and index them, then add an additional document, is it necessary to reindex the whole database? Are there any differences among various vectorstores (such as Qdrant, Chroma, Weaviate) in this regard? Additionally, is it possible to add, edit, or delete a single document using LangChain?