Elasticsearch multi-match fields doesn't include query string - elasticsearch

Question

I am doing a multi-match search using the following query object:
{
_source: [
'baseline',
'cdrp',
'date',
'description',
'dev_status',
'element',
'event',
'id'
],
track_total_hits: true,
query: {
bool: {
filter: [{name: "baseline", values: ["1f.0.1.0", "1f.1.8.3"]}],
should: [
{
multi_match:{
query: "national",
fields: ["cdrp","description","narrative.*","title","cop"]
}
}
]
}
},
highlight: { fields: { '*': {} } },
sort: [],
from: 0,
size: 50
}
I'm expecting the word "national" to be found within description or narrative.* fields but only one record out of 2 returned meet my expectations. I'm trying to understand why.
elasticsearch.config.ts
"settings": {
"analysis": {
"analyzer": {
"search_synonyms": {
"tokenizer": "whitespace",
"filter": [
"graph_synonyms",
"lowercase",
"asciifolding"
],
}
}
}
},
"mappings": {
"properties": {
"description": {
"type": "text",
"analyzer": "search_synonyms"
},
"narrative": {
"type":"object",
"properties":{
"_all":{
"type": "text",
"analyzer": "search_synonyms"
}
}
},
}
}

Answer 1

Should clause works like OR, it doesn't filter out documents it affects scoring. Documents which match should clause are scored higher.
If you want to filter on multi-match you can move it inside filter clause
filter: [
{
name: "baseline", values: ["1f.0.1.0", "1f.1.8.3"]
},
{
multi_match:
{
query: "national",
fields: ["cdrp","description","narrative.*","title","cop"]
}
}
]
Filter vs Must:- Both return documents matching clauses specified. Filter doesn't score documents. So if you are not interested in score of documents or are not concerned with order of documents returned, you can use filter. So both are same with difference of scoring.
Documents with more matches are scored higher
Multi_match by default uses best_fields
Finds documents which match any field, but uses the _score from the
best field.
It uses score returned for field with maximum number of matches to calculate score for each document.
Example
Document 1 has matches in two field , field1 (score 2), field2 (score 1)
Document 2 has matches in one field , field2 (score 3)
Documnet 2 will be ranked higher even if 1 field has matched.
You can change it to most_fields
Finds documents which match any field and combines the _score from
each field.
{
"query": {
"bool": {
"must": [
{
"multi_match": {
"query": "test",
"fields": [],
"type": "most_fields"
}
}
]
}
}
}
Still a document with fewer number of fields matched can be ranked higher due to high score in a field caused by multiple terms.
If you want to give same score to a single field irrespective of number of tokens matched. You need to use constant_score query
{
"query": {
"bool": {
"should": [
{
"constant_score": {
"filter": {
"term": {
"field1": "test"
}
}
}
},
{
"constant_score": {
"filter": {
"term": {
"field2": "test"
}
}
}
}
]
}
},
"highlight": {
"fields": {
"field1": {},
"field2": {}
}
}
}
Result:
"hits" : [
{
"_index" : "index18",
"_type" : "_doc",
"_id" : "iSCe6nEB8J88APx3YBGn",
"_score" : 2.0, --> one score per field matched
"_source" : {
"field1" : "test",
"field2" : "test"
},
"highlight" : {
"field1" : [
"<em>test</em>"
],
"field2" : [
"<em>test</em>"
]
}
},
{
"_index" : "index18",
"_type" : "_doc",
"_id" : "iiCe6nEB8J88APx3ghF-",
"_score" : 1.0,
"_source" : {
"field1" : "test",
"field2" : "abc"
},
"highlight" : {
"field1" : [
"<em>test</em>"
]
}
},
{
"_index" : "index18",
"_type" : "_doc",
"_id" : "iyCf6nEB8J88APx3UhF8",
"_score" : 1.0,
"_source" : {
"field1" : "test do",
"field2" : "abc"
},
"highlight" : {
"field1" : [
"<em>test</em> do"
]
}
}
]
}