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Highlighting

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Highlighting

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Highlighters enable you to get highlighted snippets from one or more fields in your search results so you can show users where the query matches are. When you request highlights, the response contains an additional highlight element for each search hit that includes the highlighted fields and the highlighted fragments.

Highlighting requires the actual content of a field. If the field is not stored (the mapping does not set store to true), the actual _source is loaded and the relevant field is extracted from _source.

The _all field cannot be extracted from _source, so it can only be used for highlighting if it is explicitly stored.

For example, to get highlights for the content field in each search hit using the default highlighter, include a highlight object in the request body that specifies the content field:

GET /_search
{
    "query" : {
        "match": { "content": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "content" : {}
        }
    }
}

Elasticsearch supports three highlighters: unified, plain, and fvh (fast vector highlighter). You can specify the highlighter type you want to use for each field.

Unified highlighter

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The unified highlighter uses the Lucene Unified Highlighter. This highlighter breaks the text into sentences and uses the BM25 algorithm to score individual sentences as if they were documents in the corpus. It also supports accurate phrase and multi-term (fuzzy, prefix, regex) highlighting. This is the default highlighter.

Plain highlighter

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The plain highlighter uses the standard Lucene highlighter. It attempts to reflect the query matching logic in terms of understanding word importance and any word positioning criteria in phrase queries.

The plain highlighter works best for highlighting simple query matches in a single field. To accurately reflect query logic, it creates a tiny in-memory index and re-runs the original query criteria through Lucene’s query execution planner to get access to low-level match information for the current document. This is repeated for every field and every document that needs to be highlighted. If you want to highlight a lot of fields in a lot of documents with complex queries, we recommend using the unified highlighter on postings or term_vector fields.

Fast vector highlighter

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The fvh highlighter uses the Lucene Fast Vector highlighter. This highlighter can be used on fields with term_vector set to with_positions_offsets in the mapping. The fast vector highlighter:

Can be customized with a boundary_scanner.
Requires setting term_vector to with_positions_offsets which increases the size of the index
Can combine matches from multiple fields into one result. See matched_fields
Can assign different weights to matches at different positions allowing for things like phrase matches being sorted above term matches when highlighting a Boosting Query that boosts phrase matches over term matches

Offsets Strategy

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To create meaningful search snippets from the terms being queried, the highlighter needs to know the start and end character offsets of each word in the original text. These offsets can be obtained from:

The postings list. If index_options is set to offsets in the mapping, the unified highlighter uses this information to highlight documents without re-analyzing the text. It re-runs the original query directly on the postings and extracts the matching offsets from the index, limiting the collection to the highlighted documents. This is important if you have large fields because it doesn’t require reanalyzing the text to be highlighted. It also requires less disk space than using term_vectors.
Term vectors. If term_vector information is provided by setting term_vector to with_positions_offsets in the mapping, the unified highlighter automatically uses the term_vector to highlight the field. It’s fast especially for large fields (> 1MB) and for highlighting multi-term queries like prefix or wildcard because it can access the dictionary of terms for each document. The fvh highlighter always uses term vectors.
Plain highlighting. This mode is used by the unified when there is no other alternative. It creates a tiny in-memory index and re-runs the original query criteria through Lucene’s query execution planner to get access to low-level match information on the current document. This is repeated for every field and every document that needs highlighting. The plain highlighter always uses plain highlighting.

Highlighting Settings

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Highlighting settings can be set on a global level and overridden at the field level.

boundary_chars: A string that contains each boundary character. Defaults to .,!? \t\n.
boundary_max_scan: How far to scan for boundary characters. Defaults to 20.

boundary_scanner

Specifies how to break the highlighted fragments: chars, sentence, or word. Only valid for the unified and fvh highlighters. Defaults to sentence for the unified highlighter. Defaults to chars for the fvh highlighter.

chars: Use the characters specified by boundary_chars as highlighting boundaries. The boundary_max_scan setting controls how far to scan for boundary characters. Only valid for the fvh highlighter.
sentence: Break highlighted fragments at the next sentence boundary, as determined by Java’s BreakIterator. You can specify the locale to use with boundary_scanner_locale.

When used with the unified highlighter, the sentence scanner splits sentences bigger than fragment_size at the first word boundary next to fragment_size. You can set fragment_size to 0 to never split any sentence.
word: Break highlighted fragments at the next word boundary, as determined by Java’s BreakIterator. You can specify the locale to use with boundary_scanner_locale.

boundary_scanner_locale

Controls which locale is used to search for sentence and word boundaries.

encoder

Indicates if the snippet should be HTML encoded: default (no encoding) or html (HTML-escape the snippet text and then insert the highlighting tags)

fields

Specifies the fields to retrieve highlights for. You can use wildcards to specify fields. For example, you could specify comment_* to get highlights for all text and keyword fields that start with comment_.

Only text and keyword fields are highlighted when you use wildcards. If you use a custom mapper and want to highlight on a field anyway, you must explicitly specify that field name.

force_source

Highlight based on the source even if the field is stored separately. Defaults to false.

fragmenter

Specifies how text should be broken up in highlight snippets: simple or span. Only valid for the plain highlighter. Defaults to span.

simple: Breaks up text into same-sized fragments.
span: Breaks up text into same-sized fragments, but tried to avoid breaking up text between highlighted terms. This is helpful when you’re querying for phrases. Default.

fragment_offset

Controls the margin from which you want to start highlighting. Only valid when using the fvh highlighter.

fragment_size

The size of the highlighted fragment in characters. Defaults to 100.

highlight_query

Highlight matches for a query other than the search query. This is especially useful if you use a rescore query because those are not taken into account by highlighting by default.

Elasticsearch does not validate that highlight_query contains the search query in any way so it is possible to define it so legitimate query results are not highlighted. Generally, you should include the search query as part of the highlight_query.

matched_fields

Combine matches on multiple fields to highlight a single field. This is most intuitive for multifields that analyze the same string in different ways. All matched_fields must have term_vector set to with_positions_offsets, but only the field to which the matches are combined is loaded so only that field benefits from having store set to yes. Only valid for the fvh highlighter.

no_match_size

The amount of text you want to return from the beginning of the field if there are no matching fragments to highlight. Defaults to 0 (nothing is returned).

number_of_fragments

The maximum number of fragments to return. If the number of fragments is set to 0, no fragments are returned. Instead, the entire field contents are highlighted and returned. This can be handy when you need to highlight short texts such as a title or address, but fragmentation is not required. If number_of_fragments is 0, fragment_size is ignored. Defaults to 5.

order

Sorts highlighted fragments by score when set to score. Only valid for the unified highlighter.

phrase_limit

Controls the number of matching phrases in a document that are considered. Prevents the fvh highlighter from analyzing too many phrases and consuming too much memory. When using matched_fields, `phrase_limit phrases per matched field are considered. Raising the limit increases query time and consumes more memory. Only supported by the fvh highlighter. Defaults to 256.

pre_tags

Use in conjunction with post_tags to define the HTML tags to use for the highlighted text. By default, highlighted text is wrapped in  and  tags. Specify as an array of strings.

post_tags

Use in conjunction with pre_tags to define the HTML tags to use for the highlighted text. By default, highlighted text is wrapped in  and  tags. Specify as an array of strings.

require_field_match

By default, only fields that contains a query match are highlighted. Set require_field_match to false to highlight all fields. Defaults to true.

tags_schema

Set to styled to use the built-in tag schema. The styled schema defines the following pre_tags and defines post_tags as .

<em class="hlt1">, <em class="hlt2">, <em class="hlt3">,
<em class="hlt4">, <em class="hlt5">, <em class="hlt6">,
<em class="hlt7">, <em class="hlt8">, <em class="hlt9">,
<em class="hlt10">

type: The highlighter to use: unified, plain, or fvh. Defaults to unified.

Highlighting Examples

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Override global settings

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You can specify highlighter settings globally and selectively override them for individual fields.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "number_of_fragments" : 3,
        "fragment_size" : 150,
        "fields" : {
            "_all" : { "pre_tags" : ["<em>"], "post_tags" : ["</em>"] },
            "blog.title" : { "number_of_fragments" : 0 },
            "blog.author" : { "number_of_fragments" : 0 },
            "blog.comment" : { "number_of_fragments" : 5, "order" : "score" }
        }
    }
}

Specify a highlight query

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You can specify a highlight_query to take additional information into account when highlighting. For example, the following query includes both the search query and rescore query in the highlight_query. Without the highlight_query, highlighting would only take the search query into account.

GET /_search
{
    "stored_fields": [ "_id" ],
    "query" : {
        "match": {
            "comment": {
                "query": "foo bar"
            }
        }
    },
    "rescore": {
        "window_size": 50,
        "query": {
            "rescore_query" : {
                "match_phrase": {
                    "comment": {
                        "query": "foo bar",
                        "slop": 1
                    }
                }
            },
            "rescore_query_weight" : 10
        }
    },
    "highlight" : {
        "order" : "score",
        "fields" : {
            "comment" : {
                "fragment_size" : 150,
                "number_of_fragments" : 3,
                "highlight_query": {
                    "bool": {
                        "must": {
                            "match": {
                                "comment": {
                                    "query": "foo bar"
                                }
                            }
                        },
                        "should": {
                            "match_phrase": {
                                "comment": {
                                    "query": "foo bar",
                                    "slop": 1,
                                    "boost": 10.0
                                }
                            }
                        },
                        "minimum_should_match": 0
                    }
                }
            }
        }
    }
}

Set highlighter type

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The type field allows to force a specific highlighter type. The allowed values are: unified, plain and fvh. The following is an example that forces the use of the plain highlighter:

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "comment" : {"type" : "plain"}
        }
    }
}

Configure highlighting tags

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By default, the highlighting will wrap highlighted text in  and . This can be controlled by setting pre_tags and post_tags, for example:

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "pre_tags" : ["<tag1>"],
        "post_tags" : ["</tag1>"],
        "fields" : {
            "_all" : {}
        }
    }
}

When using the fast vector highlighter, you can specify additional tags and the "importance" is ordered.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "pre_tags" : ["<tag1>", "<tag2>"],
        "post_tags" : ["</tag1>", "</tag2>"],
        "fields" : {
            "_all" : {}
        }
    }
}

You can also use the built-in styled tag schema:

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "tags_schema" : "styled",
        "fields" : {
            "comment" : {}
        }
    }
}

Highlight on source

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Forces the highlighting to highlight fields based on the source even if fields are stored separately. Defaults to false.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "comment" : {"force_source" : true}
        }
    }
}

Highlight in all fields

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By default, only fields that contains a query match are highlighted. Set require_field_match to false to highlight all fields.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "require_field_match": false,
        "fields": {
                "_all" : { "pre_tags" : ["<em>"], "post_tags" : ["</em>"] }
        }
    }
}

Combine matches on multiple fields

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This is only supported by the fvh highlighter

The Fast Vector Highlighter can combine matches on multiple fields to highlight a single field. This is most intuitive for multifields that analyze the same string in different ways. All matched_fields must have term_vector set to with_positions_offsets but only the field to which the matches are combined is loaded so only that field would benefit from having store set to yes.

In the following examples, comment is analyzed by the english analyzer and comment.plain is analyzed by the standard analyzer.

GET /_search
{
    "query": {
        "query_string": {
            "query": "comment.plain:running scissors",
            "fields": ["comment"]
        }
    },
    "highlight": {
        "order": "score",
        "fields": {
            "comment": {
                "matched_fields": ["comment", "comment.plain"],
                "type" : "fvh"
            }
        }
    }
}

The above matches both "run with scissors" and "running with scissors" and would highlight "running" and "scissors" but not "run". If both phrases appear in a large document then "running with scissors" is sorted above "run with scissors" in the fragments list because there are more matches in that fragment.

GET /_search
{
    "query": {
        "query_string": {
            "query": "running scissors",
            "fields": ["comment", "comment.plain^10"]
        }
    },
    "highlight": {
        "order": "score",
        "fields": {
            "comment": {
                "matched_fields": ["comment", "comment.plain"],
                "type" : "fvh"
            }
        }
    }
}

The above highlights "run" as well as "running" and "scissors" but still sorts "running with scissors" above "run with scissors" because the plain match ("running") is boosted.

GET /_search
{
    "query": {
        "query_string": {
            "query": "running scissors",
            "fields": ["comment", "comment.plain^10"]
        }
    },
    "highlight": {
        "order": "score",
        "fields": {
            "comment": {
                "matched_fields": ["comment.plain"],
                "type" : "fvh"
            }
        }
    }
}

The above query wouldn’t highlight "run" or "scissor" but shows that it is just fine not to list the field to which the matches are combined (comment) in the matched fields.

Technically it is also fine to add fields to matched_fields that don’t share the same underlying string as the field to which the matches are combined. The results might not make much sense and if one of the matches is off the end of the text then the whole query will fail.

There is a small amount of overhead involved with setting matched_fields to a non-empty array so always prefer

    "highlight": {
        "fields": {
            "comment": {}
        }
    }

    "highlight": {
        "fields": {
            "comment": {
                "matched_fields": ["comment"],
                "type" : "fvh"
            }
        }
    }

Explicitly order highlighted fields

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Elasticsearch highlights the fields in the order that they are sent, but per the JSON spec, objects are unordered. If you need to be explicit about the order in which fields are highlighted specify the fields as an array:

GET /_search
{
    "highlight": {
        "fields": [
            { "title": {} },
            { "text": {} }
        ]
    }
}

None of the highlighters built into Elasticsearch care about the order that the fields are highlighted but a plugin might.

Control highlighted fragments

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Each field highlighted can control the size of the highlighted fragment in characters (defaults to 100), and the maximum number of fragments to return (defaults to 5). For example:

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "comment" : {"fragment_size" : 150, "number_of_fragments" : 3}
        }
    }
}

On top of this it is possible to specify that highlighted fragments need to be sorted by score:

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "order" : "score",
        "fields" : {
            "comment" : {"fragment_size" : 150, "number_of_fragments" : 3}
        }
    }
}

If the number_of_fragments value is set to 0 then no fragments are produced, instead the whole content of the field is returned, and of course it is highlighted. This can be very handy if short texts (like document title or address) need to be highlighted but no fragmentation is required. Note that fragment_size is ignored in this case.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "_all" : {},
            "blog.title" : {"number_of_fragments" : 0}
        }
    }
}

When using fvh one can use fragment_offset parameter to control the margin to start highlighting from.

In the case where there is no matching fragment to highlight, the default is to not return anything. Instead, we can return a snippet of text from the beginning of the field by setting no_match_size (default 0) to the length of the text that you want returned. The actual length may be shorter or longer than specified as it tries to break on a word boundary.

GET /_search
{
    "query" : {
        "match": { "user": "kimchy" }
    },
    "highlight" : {
        "fields" : {
            "comment" : {
                "fragment_size" : 150,
                "number_of_fragments" : 3,
                "no_match_size": 150
            }
        }
    }
}

Highlight using the postings list

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Here is an example of setting the comment field in the index mapping to allow for highlighting using the postings:

PUT /example
{
  "mappings": {
    "doc" : {
      "properties": {
        "comment" : {
          "type": "text",
          "index_options" : "offsets"
        }
      }
    }
  }
}

Here is an example of setting the comment field to allow for highlighting using the term_vectors (this will cause the index to be bigger):

PUT /example
{
  "mappings": {
    "doc" : {
      "properties": {
        "comment" : {
          "type": "text",
          "term_vector" : "with_positions_offsets"
        }
      }
    }
  }
}

Specify a fragmenter for the plain highlighter

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When using the plain highlighter, you can choose between the simple and span fragmenters:

GET twitter/tweet/_search
{
    "query" : {
        "match_phrase": { "message": "number 1" }
    },
    "highlight" : {
        "fields" : {
            "message" : {
                "type": "plain",
                "fragment_size" : 15,
                "number_of_fragments" : 3,
                "fragmenter": "simple"
            }
        }
    }
}

Response:

{
    ...
    "hits": {
        "total": 1,
        "max_score": 1.601195,
        "hits": [
            {
                "_index": "twitter",
                "_type": "tweet",
                "_id": "1",
                "_score": 1.601195,
                "_source": {
                    "user": "test",
                    "message": "some message with the number 1",
                    "date": "2009-11-15T14:12:12",
                    "likes": 1
                },
                "highlight": {
                    "message": [
                        " with the <em>number</em>",
                        " <em>1</em>"
                    ]
                }
            }
        ]
    }
}

GET twitter/tweet/_search
{
    "query" : {
        "match_phrase": { "message": "number 1" }
    },
    "highlight" : {
        "fields" : {
            "message" : {
                "type": "plain",
                "fragment_size" : 15,
                "number_of_fragments" : 3,
                "fragmenter": "span"
            }
        }
    }
}

Response:

{
    ...
    "hits": {
        "total": 1,
        "max_score": 1.601195,
        "hits": [
            {
                "_index": "twitter",
                "_type": "tweet",
                "_id": "1",
                "_score": 1.601195,
                "_source": {
                    "user": "test",
                    "message": "some message with the number 1",
                    "date": "2009-11-15T14:12:12",
                    "likes": 1
                },
                "highlight": {
                    "message": [
                        " with the <em>number</em> <em>1</em>"
                    ]
                }
            }
        ]
    }
}

If the number_of_fragments option is set to 0, NullFragmenter is used which does not fragment the text at all. This is useful for highlighting the entire contents of a document or field.

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