boost

Individual fields can be boosted — count more towards the relevance score — at index time, with the boost parameter as follows:

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "title": {
          "type": "string",
          "boost": 2 
        },
        "content": {
          "type": "string"
        }
      }
    }
  }
}

Matches on the title field will have twice the weight as those on the content field, which has the default boost of 1.0.

Note that a title field will usually be shorter than a content field. The default relevance calculation takes field length into account, so a short title field will have a higher natural boost than a long content field.

Why index time boosting is a bad idea

We advise against using index time boosting for the following reasons:

  • You cannot change index-time boost values without reindexing all of your documents.
  • Every query supports query-time boosting which achieves the same effect. The difference is that you can tweak the boost value without having to reindex.
  • Index-time boosts are stored as part of the norm, which is only one byte. This reduces the resolution of the field length normalization factor which can lead to lower quality relevance calculations.

The only advantage that index time boosting has is that it is copied with the value into the _all field. This means that, when querying the _all field, words that originated from the title field will have a higher score than words that originated in the content field. This functionality comes at a cost: queries on the _all field are slower when index-time boosting is used.