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Vector Search

A vector search finds the approximate or exact nearest neighbors to a given query vector.

  • In a recommendation system or search engine, you can find similar records to the one you searched.
  • In LLM and other AI applications, each data point can be represented by embeddings generated from existing models, following which the search returns the most relevant features.

Distance metrics

Distance metrics are a measure of the similarity between a pair of vectors. Currently, LanceDB supports the following metrics:

Metric Description
l2 Euclidean / L2 distance
cosine Cosine Similarity
dot Dot Production

Exhaustive search (kNN)

If you do not create a vector index, LanceDB exhaustively scans the entire vector space and computes the distance to every vector in order to find the exact nearest neighbors. This is effectively a kNN search.

import lancedb
import numpy as np

db = lancedb.connect("data/sample-lancedb")

tbl = db.open_table("my_vectors")

df = tbl.search(np.random.random((1536))) \
    .limit(10) \
    .to_list()

import * as lancedb from "vectordb";

const db = await lancedb.connect("data/sample-lancedb");
const tbl = await db.openTable("my_vectors");

const results_1 = await tbl.search(Array(1536).fill(1.2)).limit(10).execute();

By default, l2 will be used as metric type. You can specify the metric type as cosine or dot if required.

df = tbl.search(np.random.random((1536))) \
    .metric("cosine") \
    .limit(10) \
    .to_list()

const results_2 = await tbl
  .search(Array(1536).fill(1.2))
  .metricType(lancedb.MetricType.Cosine)
  .limit(10)
  .execute();

To perform scalable vector retrieval with acceptable latencies, it's common to build a vector index. While the exhaustive search is guaranteed to always return 100% recall, the approximate nature of an ANN search means that using an index often involves a trade-off between recall and latency.

See the IVF_PQ index for a deeper description of how IVF_PQ indexes work in LanceDB.

Output search results

LanceDB returns vector search results via different formats commonly used in python. Let's create a LanceDB table with a nested schema:

from datetime import datetime
import lancedb
from lancedb.pydantic import LanceModel, Vector
import numpy as np
from pydantic import BaseModel
uri = "data/sample-lancedb-nested"

class Metadata(BaseModel):
    source: str
    timestamp: datetime

class Document(BaseModel):
    content: str
    meta: Metadata

class LanceSchema(LanceModel):
    id: str
    vector: Vector(1536)
    payload: Document

# Let's add 100 sample rows to our dataset
data = [LanceSchema(
    id=f"id{i}",
    vector=np.random.randn(1536),
    payload=Document(
        content=f"document{i}", meta=Metadata(source=f"source{i % 10}", timestamp=datetime.now())
    ),
) for i in range(100)]

tbl = db.create_table("documents", data=data)

As a PyArrow table

Using to_arrow() we can get the results back as a pyarrow Table. This result table has the same columns as the LanceDB table, with the addition of an _distance column for vector search or a score column for full text search.

tbl.search(np.random.randn(1536)).to_arrow()

As a Pandas DataFrame

You can also get the results as a pandas dataframe.

tbl.search(np.random.randn(1536)).to_pandas()

While other formats like Arrow/Pydantic/Python dicts have a natural way to handle nested schemas, pandas can only store nested data as a python dict column, which makes it difficult to support nested references. So for convenience, you can also tell LanceDB to flatten a nested schema when creating the pandas dataframe.

tbl.search(np.random.randn(1536)).to_pandas(flatten=True)

If your table has a deeply nested struct, you can control how many levels of nesting to flatten by passing in a positive integer.

tbl.search(np.random.randn(1536)).to_pandas(flatten=1)

As a list of Python dicts

You can of course return results as a list of python dicts.

tbl.search(np.random.randn(1536)).to_list()

As a list of Pydantic models

We can add data using Pydantic models, and we can certainly retrieve results as Pydantic models

tbl.search(np.random.randn(1536)).to_pydantic(LanceSchema)

Note that in this case the extra _distance field is discarded since it's not part of the LanceSchema.