Cross Encoder Reranker

This re-ranker uses Cross Encoder models from sentence-transformers to rerank the search results. You can use this re-ranker by passing CrossEncoderReranker() to the rerank() method.

Note

Supported Query Types: Hybrid, Vector, FTS

import numpy
import lancedb
from lancedb.embeddings import get_registry
from lancedb.pydantic import LanceModel, Vector
from lancedb.rerankers import CrossEncoderReranker

embedder = get_registry().get("sentence-transformers").create()
db = lancedb.connect("~/.lancedb")

class Schema(LanceModel):
    text: str = embedder.SourceField()
    vector: Vector(embedder.ndims()) = embedder.VectorField()

data = [
    {"text": "hello world"},
    {"text": "goodbye world"}
    ]
tbl = db.create_table("test", schema=Schema, mode="overwrite")
tbl.add(data)
reranker = CrossEncoderReranker()

# Run vector search with a reranker
result = tbl.search("hello").rerank(reranker=reranker).to_list() 

# Run FTS search with a reranker
result = tbl.search("hello", query_type="fts").rerank(reranker=reranker).to_list()

# Run hybrid search with a reranker
tbl.create_fts_index("text", replace=True)
result = tbl.search("hello", query_type="hybrid").rerank(reranker=reranker).to_list()

Accepted Arguments

Argument	Type	Default	Description
`model_name`	`str`	`""cross-encoder/ms-marco-TinyBERT-L-6"`	The name of the reranker model to use.
`column`	`str`	`"text"`	The name of the column to use as input to the cross encoder model.
`device`	`str`	`None`	The device to use for the cross encoder model. If None, will use "cuda" if available, otherwise "cpu".
`return_score`	str	`"relevance"`	Options are "relevance" or "all". The type of score to return. If "relevance", will return only the `_relevance_score. If "all" is supported, will return relevance score along with the vector and/or fts scores depending on query type

Supported Scores for each query type

You can specify the type of scores you want the reranker to return. The following are the supported scores for each query type:

Hybrid Search

`return_score`	Status	Description
`relevance`	✅ Supported	Returns only have the `_relevance_score` column
`all`	❌ Not Supported	Returns have vector(`_distance`) and FTS(`score`) along with Hybrid Search score(`_relevance_score`)

Vector Search

`return_score`	Status	Description
`relevance`	✅ Supported	Returns only have the `_relevance_score` column
`all`	✅ Supported	Returns have vector(`_distance`) along with Hybrid Search score(`_relevance_score`)

FTS Search

`return_score`	Status	Description
`relevance`	✅ Supported	Returns only have the `_relevance_score` column
`all`	✅ Supported	Returns have FTS(`score`) along with Hybrid Search score(`_relevance_score`)