similarity

similarity

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

Only text-based field types like text and keyword support this configuration.

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.

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:

  1. resp = client.indices.create(
  2.     index="my-index-000001",
  3.     mappings={
  4.         "properties": {
  5.             "default_field": {
  6.                 "type": "text"
  7.             },
  8.             "boolean_sim_field": {
  9.                 "type": "text",
  10.                 "similarity": "boolean"
  11.             }
  12.         }
  13.     },
  14. )
  15. print(resp)
  1. response = client.indices.create(
  2.   index: 'my-index-000001',
  3.   body: {
  4.     mappings: {
  5.       properties: {
  6.         default_field: {
  7.           type: 'text'
  8.         },
  9.         boolean_sim_field: {
  10.           type: 'text',
  11.           similarity: 'boolean'
  12.         }
  13.       }
  14.     }
  15.   }
  16. )
  17. puts response
  1. const response = await client.indices.create({
  2.   index: "my-index-000001",
  3.   mappings: {
  4.     properties: {
  5.       default_field: {
  6.         type: "text",
  7.       },
  8.       boolean_sim_field: {
  9.         type: "text",
  10.         similarity: "boolean",
  11.       },
  12.     },
  13.   },
  14. });
  15. console.log(response);
  1. PUT my-index-000001
  2. {
  3.   "mappings": {
  4.     "properties": {
  5.       "default_field": { 
  6.         "type": "text"
  7.       },
  8.       "boolean_sim_field": {
  9.         "type": "text",
  10.         "similarity": "boolean" 
  11.       }
  12.     }
  13.   }
  14. }

The default_field uses the BM25 similarity.

The boolean_sim_field uses the boolean similarity.