False positive highlights

Elastic Stack Elasticsearch
1

I am using an nGram tokenizer in ES 6.1.1 and getting some weird highlights:

  • multiple adjacent character ngram highlights are not merged into one
  • tra is incorrectly highlighted in doc 9

The query auftrag matches documents 7 and 9 as expected, but in doc 9 betrag is highlighted incorrectly. That's a problem with the highlighter - if the problem was with the query doc 8 would have also been returned.

Example code

#!/usr/bin/env bash

# Example based on  
# https://www.elastic.co/guide/en/elasticsearch/guide/current/ngrams-compound-words.html
# with suggestions from from 
# https://github.com/elastic/elasticsearch/issues/21000

DELETE INDEX IF EXISTS

curl -sS -XDELETE 'localhost:9200/my_index'
printf '\n-------------\n'

CREATE NEW INDEX

curl -sS -XPUT 'localhost:9200/my_index?pretty' -H 'Content-Type: application/json' -d'
{
    "settings": {
    "analysis": {
      "analyzer": {
        "trigrams": {
          "tokenizer": "my_ngram_tokenizer",
          "filter": ["lowercase"]
        }
      },
      "tokenizer": {
        "my_ngram_tokenizer": {
          "type": "nGram",
          "min_gram": "3",
          "max_gram": "3",
          "token_chars": [
            "letter",
            "digit",
            "symbol",
            "punctuation"
          ]
        }
      }
    }
},
    "mappings": {
        "my_type": {
            "properties": {
                "text": {
                    "type":     "text",
                    "analyzer": "trigrams",
                    "term_vector": "with_positions_offsets"
                }
            }
        }
    }
}
'
printf '\n-------------\n'

POPULATE INDEX

curl -sS -XPOST 'localhost:9200/my_index/my_type/_bulk?pretty' -H 'Content-Type: application/json' -d'
{ "index": { "_id": 7 }}
{ "text": "auftragen" }
{ "index": { "_id": 8 }}
{ "text": "betrag" }
{ "index": { "_id": 9 }}
{ "text": "betrag auftragen" }
'
printf '\n-------------\n'
sleep 1  # Give ES time to index

QUERY

curl -sS -XGET 'localhost:9200/my_index/my_type/_search?pretty' -H 'Content-Type: application/json' -d'
{
    "query": {
        "match": {
            "text": {
                "query": "auftrag",
                "minimum_should_match": "100%"
            }
        }
    },
      "highlight": {
        "fields": {
          "text": {
            "fragment_size": 120,
            "type": "fvh"
          }
        }
      }
}
'

The hits I get are (abbreviated):

"hits" : [
      {
        "_id" : "9",
        "_source" : {
          "text" : "betrag auftragen"
        },
        "highlight" : {
          "text" : [
            "be<em>tra</em>g <em>auf</em><em>tra</em>gen"
          ]
        }
      },
      {
        "_id" : "7",
        "_source" : {
          "text" : "auftragen"
        },
        "highlight" : {
          "text" : [
            "<em>auf</em><em>tra</em>gen"
          ]
        }
      }
    ]

I have tried various workarounds, such as using the unified/fvh highlighter and setting all options that seemed relevant, but no luck. Any hints are greatly appreciated.

2

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