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Co-authored-by: Kathleen DeRusso <[email protected]>
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leemthompo and kderusso authored Sep 19, 2024
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Many search systems are built on two-stage retrieval pipelines.

The first stage uses cheap, fast algorithms to find a subset of possible matches.
The first stage uses cheap, fast algorithms to find a broad set of possible matches.

The second stage uses a more powerful model, often machine learning-based, to reorder the documents.
This second step is called re-ranking.
This approach balances computational costs, because the resource-intensive model is only applied to a smaller set of pre-filtered results.
Because the resource-intensive model is only applied to the smaller set of pre-filtered results, this approach returns more relevant results while still optimizing for search performance and computational costs.

{es} supports various ranking and re-ranking techniques to optimize search relevance and performance.

[float]
[[re-ranking-two-stage-pipeline]]
== Two-stage retrieval pipelines

Learn about retrieval pipelines and how re-ranking can be applied to your existing search experience.

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[[re-ranking-first-stage-pipeline]]
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==== Full-text search: BM25 scoring

{es} ranks documents based on term frequency and inverse document frequency, adjusted for document length.
BM25 is the default statistical scoring algorithm in {es} and works out-of-the-box.
https://en.wikipedia.org/wiki/Okapi_BM25[BM25] is the default statistical scoring algorithm in {es} and works out-of-the-box.

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[[re-ranking-ranking-overview-vector]]
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==== Hybrid techniques

Hybrid search techniques combine results from full-text and vector search pipelines.
{es} enables combining lexical matching (BM25) and vector search scores using the advanced <<rrf,Reciprocal Rank Fusion (RRF)>> algorithm.
{es} enables combining lexical matching (BM25) and vector search scores using the <<rrf,Reciprocal Rank Fusion (RRF)>> algorithm.

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[[re-ranking-overview-second-stage]]
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<<semantic-reranking>> uses machine learning models to reorder search results based on their semantic similarity to a query.
Models can be hosted directly in your {es} cluster, or you can use <<inference-apis,inference endpoints>> to call models provided by third-party services.
Enables out-of-the-box semantic search capabilities on existing full-text search indices.
Semantic re-ranking enables out-of-the-box semantic search capabilities on existing full-text search indices.

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[[re-ranking-overview-ltr]]
==== Learning to Rank (LTR)

<<learning-to-rank>> is for advanced users.
Train a machine learning model to build a ranking function for your search experience that updates over time.
Best suited for when you have ample training data and need highly customized relevance tuning.
Learning To Rank involves training a machine learning model to build a ranking function for your search experience that updates over time.
LTR is best suited for when you have ample training data and need highly customized relevance tuning.

include::semantic-reranking.asciidoc[]
include::learning-to-rank.asciidoc[]

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