Changing Similarities | Elasticsearch: The Definitive Guide [2.x]

原文地址: https://www.elastic.co/guide/en/elasticsearch/guide/current/changing-similarities.html, 版权归 www.elastic.co 所有

WARNING: The 2.x versions of Elasticsearch have passed their EOL dates. If you are running a 2.x version, we strongly advise you to upgrade.

This documentation is no longer maintained and may be removed. For the latest information, see the current Elasticsearch documentation.

» » »

Changing Similaritiesedit

The similarity algorithm can be set on a per-field basis. It’s just a matter of specifying the chosen algorithm in the field’s mapping:

PUT /my_index
{
  "mappings": {
    "doc": {
      "properties": {
        "title": {
          "type":       "string",
          "similarity": "BM25" 
        },
        "body": {
          "type":       "string",
          "similarity": "default" 
        }
      }
  }
}

	The `title` field uses BM25 similarity.
	The `body` field uses the default similarity (see Lucene’s Practical Scoring Function).

Currently, it is not possible to change the similarity mapping for an existing field. You would need to reindex your data in order to do that.

Configuring BM25edit

Configuring a similarity is much like configuring an analyzer. Custom similarities can be specified when creating an index. For instance:

PUT /my_index
{
  "settings": {
    "similarity": {
      "my_bm25": { 
        "type": "BM25",
        "b":    0 
      }
    }
  },
  "mappings": {
    "doc": {
      "properties": {
        "title": {
          "type":       "string",
          "similarity": "my_bm25" 
        },
        "body": {
          "type":       "string",
          "similarity": "BM25" 
        }
      }
    }
  }
}

	Create a custom similarity called `my_bm25`, based on the built-in `BM25` similarity.
	Disable field-length normalization. See Tuning BM25.
	Field `title` uses the custom similarity `my_bm25`.
	Field `body` uses the built-in similarity `BM25`.

A custom similarity can be updated by closing the index, updating the index settings, and reopening the index. This allows you to experiment with different configurations without having to reindex your documents.