Q:

With what function does Elasticsearch 7.5 calculate a default score?

With what function does Elasticsearch 7.5 calculate a default score? An explanation i found here (https://www.compose.com/articles/how-scoring-works-in-elasticsearch/), but it work only for old versions as i understand, because query norm was removed in lucene 7.0.

A:

Prior to lucene 6.X, ES was using tf/idf as its default scoring algorithm, which they changed to BM25 once they started using Lucene 6.X and higher.
ES 7.5.1 uses Lucene 8.3.1 and they are using BM25 as their default scoring algorithm.
More details about the announcement of this change and other important links are below:
BM25 announcement:- https://www.elastic.co/elasticon/conf/2016/sf/improved-text-scoring-with-bm25
BM25 details and internals :- https://speakerdeck.com/elastic/improved-text-scoring-with-bm25
How to configure different scoring algo : https://www.elastic.co/guide/en/elasticsearch/reference/current/similarity.html

