I'm interested in getting accurate scores for queries that span multiple indices with distinct document types. I understand today I can use
dfs_query_then_fetch to ensure the document frequencies are relevant to the whole corpus of documents, not just each shard.
How is this affected with the switch to BM25? Would
dfs_query_then_fetch solve the same problem on queries across indices scored by BM25? Are there other terms of the calculation that need pre-querying in BM25 like document frequency needs in TF-IDF?