Depending on the ES version, it either does TF/IDF (older versions) or BM25 (newer versions) by default. You have to explicitly disable it if you don't want it.

https://www.elastic.co/guide/en/elasticsearch/reference/curr...

I might shamelessly plug my own book, Relevant Search (http://Manning.com/books/relevant-search) which introduces relevance on Elasticsearch.

Ping me (http://o19s.com/doug) if you’d like a discount code!

here - https://www.elastic.co/guide/en/elasticsearch/reference/curr...

similarity/relevance is the mathematical difference between search and database query. Its an inexact match.

Thanks. I'll read it.

If you want to reason about these things from first principles, then I recommend the IR book, which is free online: https://nlp.stanford.edu/IR-book/