55 lines
1.6 KiB
Plaintext
55 lines
1.6 KiB
Plaintext
[[similarity]]
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=== `similarity`
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Elasticsearch allows you to configure a scoring algorithm or _similarity_ per
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field. The `similarity` setting provides a simple way of choosing a similarity
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algorithm other than the default TF/IDF, such as `BM25`.
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Similarities are mostly useful for <<string,`string`>> fields, especially
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`analyzed` string fields, but can also apply to other field types.
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Custom similarities can be configured by tuning the parameters of the built-in
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similarities. For more details about this expert options, see the
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<<index-modules-similarity,similarity module>>.
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The only similarities which can be used out of the box, without any further
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configuration are:
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`classic`::
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The Default TF/IDF algorithm used by Elasticsearch and
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Lucene. See {defguide}/practical-scoring-function.html[Lucene’s Practical Scoring Function]
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for more information.
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`BM25`::
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The Okapi BM25 algorithm.
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See {defguide}/pluggable-similarites.html[Pluggable Similarity Algorithms]
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for more information.
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The `similarity` can be set on the field level when a field is first created,
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as follows:
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[source,js]
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--------------------------------------------------
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PUT my_index
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{
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"mappings": {
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"my_type": {
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"properties": {
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"default_field": { <1>
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"type": "string"
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},
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"bm25_field": {
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"type": "string",
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"similarity": "BM25" <2>
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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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// AUTOSENSE
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<1> The `default_field` uses the `classic` similarity (ie TF/IDF).
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<2> The `bm25_field` uses the `BM25` similarity.
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