similarityedit

Elasticsearch allows you to configure a scoring algorithm or similarity per field. The similarity setting provides a simple way of choosing a similarity algorithm other than the default BM25, such as boolean.

Similarities are mostly useful for text fields, but can also apply to other field types.

Custom similarities can be configured by tuning the parameters of the built-in similarities. For more details about this expert options, see the similarity module.

The only similarities which can be used out of the box, without any further configuration are:

BM25
The Okapi BM25 algorithm. The algorithm used by default in Elasticsearch and Lucene. See Pluggable Similarity Algorithms for more information.
boolean
A simple boolean similarity, which is used when full-text ranking is not needed and the score should only be based on whether the query terms match or not. Boolean similarity gives terms a score equal to their query boost.

The similarity can be set on the field level when a field is first created, as follows:

PUT my_index
{
  "mappings": {
    "properties": {
      "default_field": { 
        "type": "text"
      },
      "boolean_sim_field": {
        "type": "text",
        "similarity": "boolean" 
      }
    }
  }
}

The default_field uses the BM25 similarity.

The boolean_sim_field uses the boolean similarity.