﻿---
title: Ranking and reranking
description: Many search systems are built on multi-stage retrieval pipelines. Earlier stages use cheap, fast algorithms to find a broad set of possible matches. Later...
url: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5279/solutions/search/ranking
products:
  - Elasticsearch
applies_to:
  - Elastic Cloud Serverless: Generally available
  - Elastic Stack: Generally available
---

# Ranking and reranking
Many search systems are built on multi-stage retrieval pipelines.
Earlier stages use cheap, fast algorithms to find a broad set of possible matches.
Later stages use more powerful models, often machine learning-based, to reorder the documents. This step is called re-ranking. 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.
Elasticsearch supports various ranking and re-ranking techniques to optimize search relevance and performance.

## Two-stage retrieval pipelines


### Initial retrieval


#### Full-text search: BM25 scoring

Elasticsearch ranks documents based on term frequency and inverse document frequency, adjusted for document length. [BM25](https://en.wikipedia.org/wiki/Okapi_BM25) is the default statistical scoring algorithm in Elasticsearch.

#### Vector search: similarity scoring

Vector search involves transforming data into dense or sparse vector embeddings to capture semantic meanings, and computing similarity scores for query vectors. Store vectors using `semantic_text` fields for automatic inference and vectorization or `dense_vector` and `sparse_vector` fields when you need more control over the underlying embedding model. Query vector fields with `semantic`, `knn` or `sparse_vector` queries to compute similarity scores. Refer to [semantic search](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5279/solutions/search/semantic-search) for more information.

#### Hybrid techniques

Hybrid search techniques combine results from full-text and vector search pipelines. Elasticsearch enables combining lexical matching (BM25) and vector search scores using the [Reciprocal Rank Fusion (RRF)](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/rest-apis/reciprocal-rank-fusion) algorithm.

### Re-ranking

When using the following advanced re-ranking pipelines, first-stage retrieval mechanisms effectively generate a set of candidates. These candidates are funneled into the re-ranker to perform more computationally expensive re-ranking tasks.

#### Semantic re-ranking

[*Semantic re-ranking*](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5279/solutions/search/ranking/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 Elasticsearch cluster, or you can use [inference endpoints](https://www.elastic.co/docs/api/doc/elasticsearch/group/endpoint-inference) to call models provided by third-party services. Semantic re-ranking enables out-of-the-box semantic search capabilities on existing full-text search indices.

#### Learning to Rank (LTR)

[*Learning To Rank*](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5279/solutions/search/ranking/learning-to-rank-ltr) is for advanced users. 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.