Boosting field prefix match in Elasticsearch

Question

Is there a way to boost the scores of prefix field matches over term matches later in the field? Most of the Elasticsearch/Lucene documentation seems to focus on terms rather than fields.

For example, when searching for femal*, I'd like to have Female rank higher than Microscopic examination of specimen from female. Is there a way to do this on the query side or would I need to do something like create a separate field consisting of the the first word?

Answer 1

To do this, you could e.g. use a bool-query with a should to weigh in a span_first-query which in turn has a span_multi

Here is a runnable example you can play with: https://www.found.no/play/gist/8107157

#!/bin/bash

export ELASTICSEARCH_ENDPOINT="http://localhost:9200"

# Index documents
curl -XPOST "$ELASTICSEARCH_ENDPOINT/_bulk?refresh=true" -d '
{"index":{"_index":"play","_type":"type"}}
{"title":"Female"}
{"index":{"_index":"play","_type":"type"}}
{"title":"Female specimen"}
{"index":{"_index":"play","_type":"type"}}
{"title":"Microscopic examination of specimen from female"}
'

# Do searches

# This will match all documents.
curl -XPOST "$ELASTICSEARCH_ENDPOINT/_search?pretty" -d '
{
    "query": {
        "prefix": {
            "title": {
                "prefix": "femal"
            }
        }
    }
}
'

# This matches only the two first documents.
curl -XPOST "$ELASTICSEARCH_ENDPOINT/_search?pretty" -d '
{
    "query": {
        "span_first": {
            "end": 1,
            "match": {
                "span_multi": {
                    "match": {
                        "prefix": {
                            "title": {
                                "prefix": "femal"
                            }
                        }
                    }
                }
            }
        }
    }
}
'

# This matches all, but prefers the one's with a prefix match.
# It's sufficient that either of these match, but prefer that both matches.
curl -XPOST "$ELASTICSEARCH_ENDPOINT/_search?pretty" -d '
{
    "query": {
        "bool": {
            "should": [
                {
                    "span_first": {
                        "end": 1,
                        "match": {
                            "span_multi": {
                                "match": {
                                    "prefix": {
                                        "title": {
                                            "prefix": "femal"
                                        }
                                    }
                                }
                            }
                        }
                    }
                },
                {
                    "match": {
                        "title": {
                            "query": "femal"
                        }
                    }
                }
            ]
        }
    }
}
'