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kNN Painless Scripting extensions
With the kNN plugin’s Painless Scripting extensions, you can use kNN distance functions directly in your Painless scripts to perform operations on knn_vector
fields. Painless has a strict list of allowed functions and classes per context to ensure its scripts are secure. The kNN plugin adds Painless Scripting extensions to a few of the distance functions used in kNN score script, so you can use them to customize your kNN workload.
Get started with kNN’s Painless Scripting functions
To use kNN’s Painless Scripting functions, first create an index with knn_vector
fields like in kNN score script. Once the index is created and you ingest some data, you can use the painless extensions:
GET myknnindex2/_search
{
"size": 2,
"query": {
"script_score": {
"query": {
"bool": {
"filter": {
"term": {
"color": "BLUE"
}
}
}
},
"script": {
"source": "1.0 + cosineSimilarity(params.query_value, doc[params.field])",
"params": {
"field": "my_vector",
"query_value": [9.9, 9.9]
}
}
}
}
}
field
needs to map to a knn_vector
field, and query_value
needs to be a floating point array with the same dimension as field
.
Function types
The following table describes the available painless functions the kNN plugin provides:
Function name  Function signature  Description 

l2Squared  float l2Squared (float[] queryVector, doc['vector field'])  This function calculates the square of the L2 distance (Euclidean distance) between a given query vector and document vectors. The shorter the distance, the more relevant the document is, so this example inverts the return value of the l2Squared function. If the document vector matches the query vector, the result is 0, so this example also adds 1 to the distance to avoid divide by zero errors. 
l1Norm  float l1Norm (float[] queryVector, doc['vector field'])  This function calculates the square of the L2 distance (Euclidean distance) between a given query vector and document vectors. The shorter the distance, the more relevant the document is, so this example inverts the return value of the l2Squared function. If the document vector matches the query vector, the result is 0, so this example also adds 1 to the distance to avoid divide by zero errors. 
cosineSimilarity  float cosineSimilarity (float[] queryVector, doc['vector field'])  Cosine similarity is an inner product of the query vector and document vector normalized to both have a length of 1. If the magnitude of the query vector doesn’t change throughout the query, you can pass the magnitude of the query vector to improve performance, instead of calculating the magnitude every time for every filtered document:float cosineSimilarity (float[] queryVector, doc['vector field'], float normQueryVector) In general, the range of cosine similarity is [1, 1]. However, in the case of information retrieval, the cosine similarity of two documents ranges from 0 to 1 because the tfidf statistic can’t be negative. Therefore, the kNN plugin adds 1.0 in order to always yield a positive cosine similarity score. 
Constraints

If a document’s
knn_vector
field has different dimensions than the query, the function throws anIllegalArgumentException
. 
If a vector field doesn’t have a value, the function throws an
IllegalStateException
.You can avoid this situation by first checking if a document has a value in its field:
"source": "doc[params.field].size() == 0 ? 0 : 1 / (1 + l2Squared(params.query_value, doc[params.field]))",
Because scores can only be positive, this script ranks documents with vector fields higher than those without.