388 lines
12 KiB
Plaintext
388 lines
12 KiB
Plaintext
[[search-request-highlighting]]
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=== Highlighting
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Allows to highlight search results on one or more fields. The
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implementation uses either the lucene `highlighter`, `fast-vector-highlighter`
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or `postings-highlighter`. The following is an example of the search request
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body:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"fields" : {
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"content" : {}
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}
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}
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}
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--------------------------------------------------
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In the above case, the `content` field will be highlighted for each
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search hit (there will be another element in each search hit, called
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`highlight`, which includes the highlighted fields and the highlighted
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fragments).
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In order to perform highlighting, the actual content of the field is
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required. If the field in question is stored (has `store` set to `true`
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in the mapping) it will be used, otherwise, the actual `_source` will
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be loaded and the relevant field will be extracted from it.
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The field name supports wildcard notation. For example, using `comment_*`
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will cause all fields that match the expression to be highlighted.
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==== Postings highlighter
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If `index_options` is set to `offsets` in the mapping the postings highlighter
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will be used instead of the plain highlighter. The postings highlighter:
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* Is faster since it doesn't require to reanalyze the text to be highlighted:
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the larger the documents the better the performance gain should be
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* Requires less disk space than term_vectors, needed for the fast vector
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highlighter
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* Breaks the text into sentences and highlights them. Plays really well with
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natural languages, not as well with fields containing for instance html markup
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* Treats the document as the whole corpus, and scores individual sentences as
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if they were documents in this corpus, using the BM25 algorithm
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Here is an example of setting the `content` field to allow for
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highlighting using the postings highlighter on it:
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[source,js]
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--------------------------------------------------
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{
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"type_name" : {
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"content" : {"index_options" : "offsets"}
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}
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}
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--------------------------------------------------
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Note that the postings highlighter is meant to perform simple query terms
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highlighting, regardless of their positions. That means that when used for
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instance in combination with a phrase query, it will highlight all the terms
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that the query is composed of, regardless of whether they are actually part of
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a query match, effectively ignoring their positions.
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==== Fast vector highlighter
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If `term_vector` information is provided by setting `term_vector` to
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`with_positions_offsets` in the mapping then the fast vector highlighter
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will be used instead of the plain highlighter. The fast vector highlighter:
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* Is faster especially for large fields (> `1MB`)
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* Can be customized with `boundary_chars`, `boundary_max_scan`, and
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`fragment_offset` (see <<boundary-characters,below>>)
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* Requires setting `term_vector` to `with_positions_offsets` which
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increases the size of the index
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Here is an example of setting the `content` field to allow for
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highlighting using the fast vector highlighter on it (this will cause
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the index to be bigger):
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[source,js]
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--------------------------------------------------
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{
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"type_name" : {
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"content" : {"term_vector" : "with_positions_offsets"}
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}
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}
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--------------------------------------------------
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==== Force highlighter type
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The `type` field allows to force a specific highlighter type. This is useful
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for instance when needing to use the plain highlighter on a field that has
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`term_vectors` enabled. The allowed values are: `plain`, `postings` and `fvh`.
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The following is an example that forces the use of the plain highlighter:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"fields" : {
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"content" : { "type" : "plain"}
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}
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}
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}
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--------------------------------------------------
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[[tags]]
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==== Highlighting Tags
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By default, the highlighting will wrap highlighted text in `<em>` and
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`</em>`. This can be controlled by setting `pre_tags` and `post_tags`,
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for example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"pre_tags" : ["<tag1>"],
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"post_tags" : ["</tag1>"],
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"fields" : {
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"_all" : {}
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}
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}
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}
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--------------------------------------------------
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Using the fast vector highlighter there can be more tags, and the "importance"
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is ordered.
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"pre_tags" : ["<tag1>", "<tag2>"],
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"post_tags" : ["</tag1>", "</tag2>"],
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"fields" : {
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"_all" : {}
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}
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}
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}
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--------------------------------------------------
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There are also built in "tag" schemas, with currently a single schema
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called `styled` with the following `pre_tags`:
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[source,js]
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--------------------------------------------------
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<em class="hlt1">, <em class="hlt2">, <em class="hlt3">,
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<em class="hlt4">, <em class="hlt5">, <em class="hlt6">,
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<em class="hlt7">, <em class="hlt8">, <em class="hlt9">,
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<em class="hlt10">
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--------------------------------------------------
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and `</em>` as `post_tags`. If you think of more nice to have built in tag
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schemas, just send an email to the mailing list or open an issue. Here
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is an example of switching tag schemas:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"tags_schema" : "styled",
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"fields" : {
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"content" : {}
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}
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}
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}
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--------------------------------------------------
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==== Encoder
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An `encoder` parameter can be used to define how highlighted text will
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be encoded. It can be either `default` (no encoding) or `html` (will
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escape html, if you use html highlighting tags).
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==== Highlighted Fragments
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Each field highlighted can control the size of the highlighted fragment
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in characters (defaults to `100`), and the maximum number of fragments
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to return (defaults to `5`).
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For example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"fields" : {
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"content" : {"fragment_size" : 150, "number_of_fragments" : 3}
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}
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}
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}
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--------------------------------------------------
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The `fragment_size` is ignored when using the postings highlighter, as it
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outputs sentences regardless of their length.
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On top of this it is possible to specify that highlighted fragments need
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to be sorted by score:
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"order" : "score",
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"fields" : {
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"content" : {"fragment_size" : 150, "number_of_fragments" : 3}
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}
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}
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}
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--------------------------------------------------
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If the `number_of_fragments` value is set to `0` then no fragments are
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produced, instead the whole content of the field is returned, and of
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course it is highlighted. This can be very handy if short texts (like
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document title or address) need to be highlighted but no fragmentation
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is required. Note that `fragment_size` is ignored in this case.
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"fields" : {
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"_all" : {},
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"bio.title" : {"number_of_fragments" : 0}
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}
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}
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}
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--------------------------------------------------
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When using `fast-vector-highlighter` one can use `fragment_offset`
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parameter to control the margin to start highlighting from.
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In the case where there is no matching fragment to highlight, the default is
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to not return anything. Instead, we can return a snippet of text from the
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beginning of the field by setting `no_match_size` (default `0`) to the length
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of the text that you want returned. The actual length may be shorter than
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specified as it tries to break on a word boundary. When using the postings
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highlighter it is not possible to control the actual size of the snippet,
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therefore the first sentence gets returned whenever `no_match_size` is
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greater than `0`.
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"fields" : {
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"content" : {
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"fragment_size" : 150,
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"number_of_fragments" : 3,
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"no_match_size": 150
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}
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}
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}
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}
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--------------------------------------------------
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==== Highlight query
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It is also possible to highlight against a query other than the search
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query by setting `highlight_query`. This is especially useful if you
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use a rescore query because those are not taken into account by
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highlighting by default. Elasticsearch does not validate that
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`highlight_query` contains the search query in any way so it is possible
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to define it so legitimate query results aren't highlighted at all.
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Generally it is better to include the search query in the
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`highlight_query`. Here is an example of including both the search
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query and the rescore query in `highlight_query`.
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[source,js]
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--------------------------------------------------
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{
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"fields": [ "_id" ],
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"query" : {
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"match": {
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"content": {
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"query": "foo bar"
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}
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}
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},
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"rescore": {
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"window_size": 50,
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"query": {
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"rescore_query" : {
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"match_phrase": {
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"content": {
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"query": "foo bar",
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"phrase_slop": 1
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}
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}
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},
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"rescore_query_weight" : 10
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}
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},
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"highlight" : {
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"order" : "score",
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"fields" : {
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"content" : {
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"fragment_size" : 150,
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"number_of_fragments" : 3,
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"highlight_query": {
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"bool": {
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"must": {
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"match": {
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"content": {
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"query": "foo bar"
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}
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}
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},
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"should": {
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"match_phrase": {
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"content": {
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"query": "foo bar",
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"phrase_slop": 1,
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"boost": 10.0
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}
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}
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},
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"minimum_should_match": 0
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}
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}
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}
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}
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}
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}
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--------------------------------------------------
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Note that the score of text fragment in this case is calculated by the Lucene
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highlighting framework. For implementation details you can check the
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`ScoreOrderFragmentsBuilder.java` class. On the other hand when using the
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postings highlighter the fragments are scored using, as mentioned above,
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the BM25 algorithm.
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[[highlighting-settings]]
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==== Global Settings
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Highlighting settings can be set on a global level and then overridden
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at the field level.
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[source,js]
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--------------------------------------------------
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{
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"query" : {...},
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"highlight" : {
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"number_of_fragments" : 3,
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"fragment_size" : 150,
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"tag_schema" : "styled",
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"fields" : {
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"_all" : { "pre_tags" : ["<em>"], "post_tags" : ["</em>"] },
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"bio.title" : { "number_of_fragments" : 0 },
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"bio.author" : { "number_of_fragments" : 0 },
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"bio.content" : { "number_of_fragments" : 5, "order" : "score" }
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}
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}
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}
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--------------------------------------------------
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[[field-match]]
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==== Require Field Match
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`require_field_match` can be set to `true` which will cause a field to
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be highlighted only if a query matched that field. `false` means that
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terms are highlighted on all requested fields regardless if the query
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matches specifically on them.
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[[boundary-characters]]
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==== Boundary Characters
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When highlighting a field using the fast vector highlighter,
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`boundary_chars` can be configured to define what constitutes a boundary
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for highlighting. It's a single string with each boundary character
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defined in it. It defaults to `.,!? \t\n`.
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The `boundary_max_scan` allows to control how far to look for boundary
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characters, and defaults to `20`.
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