How Elasticsearch relevance score gets calculated? - elasticsearch

Question

I am using multi_match with phrase_prefix for full text search in Elasticsearch 5.5. ES query looks like
{
query: {
bool: {
must: {
multi_match: {
query: "butt",
type: "phrase_prefix",
fields: ["item.name", "item.keywords"],
max_expansions: 10
}
}
}
}
}
I am getting following response
[
{
"_index": "items_index",
"_type": "item",
"_id": "2",
"_score": 0.61426216,
"_source": {
"item": {
"keywords": "amul butter, milk, butter milk, flavoured",
"name": "Flavoured Butter"
}
}
},
{
"_index": "items_index",
"_type": "item",
"_id": "1",
"_score": 0.39063013,
"_source": {
"item": {
"keywords": "amul butter, milk, butter milk",
"name": "Butter Milk"
}
}
}
]
Mappings is as follows(I am using default mappings)
{
"items_index" : {
"mappings" : {
"parent_doc": {
...
"properties": {
"item" : {
"properties" : {
"keywords" : {
"type" : "text",
"fields" : {
"keyword" : {
"type" : "keyword",
"ignore_above" : 256
}
}
},
"name" : {
"type" : "text",
"fields" : {
"keyword" : {
"type" : "keyword",
"ignore_above" : 256
}
}
}
}
}
}
}
}
}
How item with "name": "Flavoured Butter" getting higher score of 0.61426216 against the document with "name": "Butter Milk" and score 0.39063013?
I tried applying boost to "item.name" and removing "item.keywords" form search fields getting same results.
How scores in Elasticsearch works? Are above results correct in terms of relavance?

Answer 1

The scoring for phrase_prefix is similar to that of best_fields, meaning that score of a document is the score obtained from the best_field, which here is item.keywords.
So, item.name isn't adding to score
Refer: multi-match-types
You can use 2 multi_match queries to combine the score from keywords and name.
{
"query": {
"bool": {
"must": [{
"multi_match": {
"query": "butt",
"type": "phrase_prefix",
"fields": [
"item.keywords"
],
"max_expansions": 10
}
},{
"multi_match": {
"query": "butt",
"type": "phrase_prefix",
"fields": [
"item.name"
],
"max_expansions": 10
}
}]
}
}
}