135 lines
3.9 KiB
Plaintext
135 lines
3.9 KiB
Plaintext
[[index-modules-similarity]]
|
|
== Similarity module
|
|
|
|
A similarity (scoring / ranking model) defines how matching documents
|
|
are scored. Similarity is per field, meaning that via the mapping one
|
|
can define a different similarity per field.
|
|
|
|
Configuring a custom similarity is considered a expert feature and the
|
|
builtin similarities are most likely sufficient as is described in the
|
|
<<mapping-core-types,mapping section>>
|
|
|
|
[float]
|
|
=== Configuring a similarity
|
|
|
|
Most existing or custom Similarities have configuration options which
|
|
can be configured via the index settings as shown below. The index
|
|
options can be provided when creating an index or updating index
|
|
settings.
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
"similarity" : {
|
|
"my_similarity" : {
|
|
"type" : "DFR",
|
|
"basic_model" : "g",
|
|
"after_effect" : "l",
|
|
"normalization" : "h2",
|
|
"normalization.h2.c" : "3.0"
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
Here we configure the DFRSimilarity so it can be referenced as
|
|
`my_similarity` in mappings as is illustrate in the below example:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"book" : {
|
|
"properties" : {
|
|
"title" : { "type" : "string", "similarity" : "my_similarity" }
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
[float]
|
|
=== Available similarities
|
|
|
|
[float]
|
|
==== Default similarity
|
|
|
|
The default similarity that is based on the TF/IDF model. This
|
|
similarity has the following option:
|
|
|
|
`discount_overlaps`::
|
|
Determines whether overlap tokens (Tokens with
|
|
0 position increment) are ignored when computing norm. By default this
|
|
is true, meaning overlap tokens do not count when computing norms.
|
|
|
|
Type name: `default`
|
|
|
|
[float]
|
|
==== BM25 similarity
|
|
|
|
Another TF/IDF based similarity that has built-in tf normalization and
|
|
is supposed to work better for short fields (like names). See
|
|
http://en.wikipedia.org/wiki/Okapi_BM25[Okapi_BM25] for more details.
|
|
This similarity has the following options:
|
|
|
|
[horizontal]
|
|
`k1`::
|
|
Controls non-linear term frequency normalization
|
|
(saturation).
|
|
|
|
`b`::
|
|
Controls to what degree document length normalizes tf values.
|
|
|
|
`discount_overlaps`::
|
|
Determines whether overlap tokens (Tokens with
|
|
0 position increment) are ignored when computing norm. By default this
|
|
is true, meaning overlap tokens do not count when computing norms.
|
|
|
|
Type name: `BM25`
|
|
|
|
[float]
|
|
==== DRF similarity
|
|
|
|
Similarity that implements the
|
|
http://lucene.apache.org/core/4_1_0/core/org/apache/lucene/search/similarities/DFRSimilarity.html[divergence
|
|
from randomness] framework. This similarity has the following options:
|
|
|
|
[horizontal]
|
|
`basic_model`::
|
|
Possible values: `be`, `d`, `g`, `if`, `in`, `ine` and `p`.
|
|
|
|
`after_effect`::
|
|
Possible values: `no`, `b` and `l`.
|
|
|
|
`normalization`::
|
|
Possible values: `no`, `h1`, `h2`, `h3` and `z`.
|
|
|
|
All options but the first option need a normalization value.
|
|
|
|
Type name: `DFR`
|
|
|
|
[float]
|
|
==== IB similarity.
|
|
|
|
http://lucene.apache.org/core/4_1_0/core/org/apache/lucene/search/similarities/IBSimilarity.html[Information
|
|
based model] . This similarity has the following options:
|
|
|
|
[horizontal]
|
|
`distribution`:: Possible values: `ll` and `spl`.
|
|
`lambda`:: Possible values: `df` and `ttf`.
|
|
`normalization`:: Same as in `DFR` similarity.
|
|
|
|
Type name: `IB`
|
|
|
|
[float]
|
|
==== Default and Base Similarities
|
|
|
|
By default, Elasticsearch will use whatever similarity is configured as
|
|
`default`. However, the similarity functions `queryNorm()` and `coord()`
|
|
are not per-field. Consequently, for expert users wanting to change the
|
|
implementation used for these two methods, while not changing the
|
|
`default`, it is possible to configure a similarity with the name
|
|
`base`. This similarity will then be used for the two methods.
|
|
|
|
You can change the default similarity for all fields like this:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
index.similarity.default.type: BM25
|
|
--------------------------------------------------
|