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Elastic Docs ›Elasticsearch Guide [8.14] ›REST APIs ›Search APIs

Retriever

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Retriever

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This functionality is in technical preview and may be changed or removed in a future release. The syntax will likely change before GA. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.

A retriever is a specification to describe top documents returned from a search. A retriever replaces other elements of the search API that also return top documents such as query and knn. A retriever may have child retrievers where a retriever with two or more children is considered a compound retriever. This allows for complex behavior to be depicted in a tree-like structure, called the retriever tree, to better clarify the order of operations that occur during a search.

Refer to Retrievers for a high level overview of the retrievers abstraction.

The following retrievers are available:

standard
A retriever that replaces the functionality of a traditional query.
knn
A retriever that replaces the functionality of a knn search.
rrf
A retriever that produces top documents from reciprocal rank fusion (RRF).

Standard Retriever

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A standard retriever returns top documents from a traditional query.

Parameters:

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query

(Optional, query object)

Defines a query to retrieve a set of top documents.

filter

(Optional, query object or list of query objects)

Applies a boolean query filter to this retriever where all documents must match this query but do not contribute to the score.

search_after

(Optional, search after object)

Defines a search after object parameter used for pagination.

terminate_after

(Optional, integer) Maximum number of documents to collect for each shard. If a query reaches this limit, Elasticsearch terminates the query early. Elasticsearch collects documents before sorting.

Use with caution. Elasticsearch applies this parameter to each shard handling the request. When possible, let Elasticsearch perform early termination automatically. Avoid specifying this parameter for requests that target data streams with backing indices across multiple data tiers.

sort

(Optional, sort object) A sort object that that specifies the order of matching documents.

min_score

(Optional, float)

Minimum _score for matching documents. Documents with a lower _score are not included in the top documents.

collapse

(Optional, collapse object)

Collapses the top documents by a specified key into a single top document per key.

Restrictions

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When a retriever tree contains a compound retriever (a retriever with two or more child retrievers) only the query element is allowed.

Example

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GET /index/_search
{
    "retriever": {
        "standard": {
            "query" { ... },
            "filter" { ... },
            "min_score": ...
        }
    },
    "size": ...
}

kNN Retriever

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A kNN retriever returns top documents from a k-nearest neighbor search (kNN).

Parameters

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field

(Required, string)

The name of the vector field to search against. Must be a dense_vector field with indexing enabled.

query_vector

(Required if query_vector_builder is not defined, array of float)

Query vector. Must have the same number of dimensions as the vector field you are searching against. Must be either an array of floats or a hex-encoded byte vector.

query_vector_builder

(Required if query_vector is not defined, query vector builder object)

Defines a model to build a query vector.

k

(Required, integer)

Number of nearest neighbors to return as top hits. This value must be fewer than or equal to num_candidates.

num_candidates

(Required, integer)

The number of nearest neighbor candidates to consider per shard. Needs to be greater than k, or size if k is omitted, and cannot exceed 10,000. Elasticsearch collects num_candidates results from each shard, then merges them to find the top k results. Increasing num_candidates tends to improve the accuracy of the final k results. Defaults to Math.min(1.5 * k, 10_000).

filter

(Optional, query object or list of query objects)

Query to filter the documents that can match. The kNN search will return the top k documents that also match this filter. The value can be a single query or a list of queries. If filter is not provided, all documents are allowed to match.

similarity

(Optional, float)

The minimum similarity required for a document to be considered a match. The similarity value calculated relates to the raw similarity used. Not the document score. The matched documents are then scored according to similarity and the provided boost is applied.

The similarity parameter is the direct vector similarity calculation.

  • l2_norm: also known as Euclidean, will include documents where the vector is within the dims dimensional hypersphere with radius similarity with origin at query_vector.
  • cosine, dot_product, and max_inner_product: Only return vectors where the cosine similarity or dot-product are at least the provided similarity.

Read more here: knn similarity search

Restrictions

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The parameters query_vector and query_vector_builder cannot be used together.

Example:

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GET /index/_search
{
    "retriever": {
        "knn": {
            "field": ...,
            "query_vector": ...,
            "k": ...,
            "num_candidates": ...
        }
    }
}

RRF Retriever

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An RRF retriever returns top documents based on the RRF formula equally weighting two or more child retrievers.

Parameters

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retrievers

(Required, array of retriever objects)

A list of child retrievers to specify which sets of returned top documents will have the RRF formula applied to them. Each child retriever carries an equal weight as part of the RRF formula. Two or more child retrievers are required.

rank_constant

(Optional, integer)

This value determines how much influence documents in individual result sets per query have over the final ranked result set. A higher value indicates that lower ranked documents have more influence. This value must be greater than or equal to 1. Defaults to 60.

window_size

(Optional, integer)

This value determines the size of the individual result sets per query. A higher value will improve result relevance at the cost of performance. The final ranked result set is pruned down to the search request’s size. window_size must be greater than or equal to size and greater than or equal to 1. Defaults to the size parameter.

Restrictions

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An RRF retriever is a compound retriever. Child retrievers may not use elements that are restricted by having a compound retriever as part of the retriever tree.

Example

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GET /index/_search
{
    "retriever": {
        "rrf": {
            "retrievers": [
                {
                    "standard" { ... }
                },
                {
                    "knn": { ... }
                }
            ],
            "rank_constant": ...
            "window_size": ...
        }
    }
}

Using from and size with a retriever tree

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The from and size parameters are provided globally as part of the general search API. They are applied to all retrievers in a retriever tree unless a specific retriever overrides the size parameter using a different parameter such as window_size. Though, the final search hits are always limited to size.

Using aggregations with a retriever tree

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Aggregations are globally specified as part of a search request. The query used for an aggregation is the combination of all leaf retrievers as should clauses in a boolean query.

Restrictions on search parameters when specifying a retriever

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When a retriever is specified as part of a search the following elements are not allowed at the top-level and instead are only allowed as elements of specific retrievers:

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