Query-Time Boosting

In Prioritizing Clauses, we explained how you could use the boost parameter at search time to give one query clause more importance than another. For instance:

GET /_search
{
  "query": {
    "bool": {
      "should": [
        {
          "match": {
            "title": {
              "query": "quick brown fox",
              "boost": 2 
            }
          }
        },
        {
          "match": { 
            "content": "quick brown fox"
          }
        }
      ]
    }
  }
}

Query-time boosting is the main tool that you can use to tune relevance. Any type of query accepts a boost parameter. Setting a boost of 2 doesn’t simply double the final _score; the actual boost value that is applied goes through normalization and some internal optimization. However, it does imply that a clause with a boost of 2 is twice as important as a clause with a boost of 1.

Practically, there is no simple formula for deciding on the “correct” boost value for a particular query clause. It’s a matter of try-it-and-see. Remember that boost is just one of the factors involved in the relevance score; it has to compete with the other factors. For instance, in the preceding example, the title field will probably already have a “natural” boost over the content field thanks to the field-length norm (titles are usually shorter than the related content), so don’t blindly boost fields just because you think they should be boosted. Apply a boost and check the results. Change the boost and check again.

GET /docs_2014_*/_search 
{
  "indices_boost": { 
    "docs_2014_10": 3,
    "docs_2014_09": 2
  },
  "query": {
    "match": {
      "text": "quick brown fox"
    }
  }
}