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Vector Indexes in LanceDB

LanceDB offers two main vector indexing algorithms: Inverted File (IVF) Index and Hierarchically Navigable Small Worlds (HNSW) Index. You can create multiple vector indexes within a Lance table. This guide will walk you through some common configurations and build patterns.

Option 1: Self-Hosted Indexing

Manual, Sync or Async: If using LanceDB Open Source, you will have to build indexes manually, as well as reindex and tune indexing parameters. The Python SDK lets you do this sychronously and asychronously.

Option 2: Automated Indexing

Automatic and Async: Indexing is automatic in LanceDB Cloud/Enterprise. As soon as data is updated, our system automates index optimization. This is done asychronously.

Here is what happens in the background - when a table contains a single vector column named vector, LanceDB automatically:

  • Infers the vector column from the table schema
  • Creates an optimized IVF_PQ index without manual configuration
  • The default distance is l2 or euclidean

Finally, LanceDB Cloud/Enterprise will analyze your data distribution to automatically configure indexing parameters.

Manual Index Creation

You can create a new index with different parameters using create_index - this replaces any existing index

Although the create_index API returns immediately, the building of the vector index is asynchronous. To wait until all data is fully indexed, you can specify the wait_timeout parameter.

Example: Construct IVF Index

In this example, we will create an index for a table containing 1536-dimensional vectors. The index will use IVF_PQ with L2 distance, which is well-suited for high-dimensional vector search.

Make sure you have enough data in your table (at least a few thousand rows) for effective index training.

Index Configuration

Sometimes you need to configure the index beyond default parameters:

  • Index Types:
    • IVF_PQ: Default index type, optimized for high-dimensional vectors
    • IVF_HNSW_SQ: Combines IVF clustering with HNSW graph for improved search quality
  • metrix: default is l2, other available are cosine or dot
    • When using cosine similarity, distances range from 0 (identical vectors) to 2 (maximally dissimilar)
  • num_partitions: The number of partitions in the IVF portion of the index. This number is usually chosen to target a particular number of vectors per partition.
  • num_sub_vectors: The number of sub-vectors that will be created during Product Quantization (PQ). This number is typically chosen based on the desired recall and the dimensionality of the vector.

Let's take a look at a sample request for an IVF index:

tbl.create_index(metric="l2", num_partitions=256, num_sub_vectors=96)
Please remember - num_partitions=256 and num_sub_vectors=96 does not work for every dataset. Those values needs to be adjusted for your particular dataset.

1. Setup

First, we connect to LanceDB and open the table we want to index.

import lancedb
from datetime import timedelta

# Connect to LanceDB
db = lancedb.connect(
  uri="db://your-project-slug",
  api_key="your-api-key",
  region="us-east-1"
)

# Use the table from quickstart
table_name = "lancedb-cloud-quickstart"
table = db.open_table(table_name)

import * as lancedb from "@lancedb/lancedb"

const db = await lancedb.connect({
  uri: "db://your-project-slug",
  apiKey: "your-api-key",
  region: "us-east-1"
});

const tableName = "myTable"
const table = await db.openTable(tableName);

2. Construct IVF Index

Next, create an IVF_PQ index with cosine similarity. You only need to specify the vector_column_name if using multiple vector columns or non-default names.

Product Quantization is the default indexing option. If you want to use Scalar Quantization, switch to index_type: IVF_SQ

table.create_index(metric="cosine", vector_column_name="keywords_embeddings", wait_timeout=timedelta(seconds=60))

await table.createIndex("keywords_embeddings", {
  config: lancedb.Index.ivfPq({
    distanceType: 'cosine'
  })
});

3. Query IVF Index

Search using a random 1536-dimensional embedding:

tbl.search(np.random.random((1536))) \
    .limit(2) \
    .nprobes(20) \
    .refine_factor(10) \
    .to_pandas()

Search Configuration

The above query will perform a search on the table tbl using the given query vector, with the following parameters:

  • limit: The number of results to return
  • nprobes: The number of probes determines the distribution of vector space. While a higher number enhances search accuracy, it also results in slower performance. Typically, setting nprobes to cover 5–10% of the dataset proves effective in achieving high recall with minimal latency.
  • refine_factor: Refine the results by reading extra elements and re-ranking them in memory. A higher number makes the search more accurate but also slower (see the FAQ page for more details on this).
  • to_pandas(): Convert the results to a pandas DataFrame

Example: Construct HNSW Index

Index Configuration

There are three key parameters to set when constructing an HNSW index:

  • metric: The default is l2 euclidean distance metric. Other available are dot and cosine.
  • m: The number of neighbors to select for each vector in the HNSW graph.
  • ef_construction: The number of candidates to evaluate during the construction of the HNSW graph.

1. Construct HNSW Index

tbl.create_index(index_type=IVF_HNSW_SQ)

2. Query HNSW Index

Search using a random 1536-dimensional embedding

tbl.search(np.random.random((1536))) \
    .limit(2) \
    .to_pandas()

Example: Construct the Binary Vector Index

Binary vectors are useful for hash-based retrieval, fingerprinting, or any scenario where data can be represented as bits.

Index Configuration

Binary vectors should be stored as fixed-size binary data (uint8 arrays, with 8 bits per byte). For storage, pack binary vectors into bytes to save space.

  • Index Type: IVF_FLAT is used for indexing binary vectors
  • metric: the hamming distance is used for similarity search
  • The dimension of binary vectors must be a multiple of 8. For example, a 128-dimensional vector is stored as a uint8 array of size 16.

1. Create Table and Schema

First, we create a table with a schema that includes a binary vector field, where each vector is stored as a packed array of bits.

table_name = "test-hamming"
ndim = 256
schema = pa.schema([
    pa.field("id", pa.int64()),
    # For dim=256, store every 8 bits in a byte (32 bytes total)
    pa.field("vector", pa.list_(pa.uint8(), 32)),
])

table = db.create_table(table_name, schema=schema, mode="overwrite")

const binaryTableName = "test-hamming-ts";
const ndim = 256;
const bytesPerVector = ndim / 8; // 32 bytes for 256 bits

const binarySchema = new Schema([
  new Field("id", new Int32(), true),
  new Field("vector", new FixedSizeList(32, new Field("item", new Uint8()))),
]);

2. Generate and Add Data

Next, we generate random binary vectors and add them to our table, packing the bits to save space.

data = []
for i in range(1024):
    vector = np.random.randint(0, 2, size=ndim)
    vector = np.packbits(vector)  # Optional: pack bits to save space
    data.append({"id": i, "vector": vector})
table.add(data)

const data = makeArrowTable(
  Array(1_000).fill(0).map((_, i) => ({
    id: i,
    vector: packBits(Array(ndim).fill(0).map(() => Math.floor(Math.random() * 2))),
  })),
  { schema: binarySchema },
);

// Create and populate table
const table = await db.createTable(binaryTableName, data);
console.log(`Created table: ${binaryTableName}`);

3. Construct Binary Index

We create an IVF_FLAT index optimized for hamming distance calculations on binary vectors.

table.create_index(
    metric="hamming",
    vector_column_name="vector",
    index_type="IVF_FLAT"
)

console.log("Creating binary vector index...");
await table.createIndex("vector", {
  config: Index.ivfFlat({
    distanceType: "hamming",
  }),
});

// Wait for index
const binaryIndexName = "vector_idx";
await table.waitForIndex([binaryIndexName], 60)

Finally, we perform a search using a random query vector, finding the nearest neighbors based on Hamming distance.

query = np.random.randint(0, 2, size=256) query = np.packbits(query) df = table.search(query).metric("hamming").limit(10).to_pandas() df.vector = df.vector.apply(np.unpackbits) ```

const query = packBits(Array(ndim).fill(0).map(() => Math.floor(Math.random() * 2)));
console.log("Performing binary vector search...");
const binaryResults = await table
  .query()
  .nearestTo(query)
  .limit(10)
  .toArray();

// Unpack vectors for display
const unpackedResults = binaryResults.map(row => ({
  ...row,
  vector: Array.from(row.vector)
}));
console.log("Binary vector search results:", unpackedResults);

Check Index Status

Vector index creation is fast - typically a few minutes for 1 million vectors with 1536 dimensions. You can check index status in two ways:

Option 1: Check the UI

Navigate to your table page - the "Index" column shows index status. It remains blank if no index exists or if creation is in progress. index_status

Option 2: Use the API

Use list_indices() and index_stats() to check index status. The index name is formed by appending "_idx" to the column name. Note that list_indices() only returns information after the index is fully built. To wait until all data is fully indexed, you can specify the wait_timeout parameter on create_index() or call wait_for_index() on the table.

import time

index_name = "keywords_embeddings_idx"
table.wait_for_index([index_name])
print(table.index_stats(index_name))
# IndexStatistics(num_indexed_rows=3000, num_unindexed_rows=0, index_type='IVF_PQ', 
#   distance_type='cosine', num_indices=None)

const indexName = "keywords_embeddings_idx"
await table.waitForIndex([indexName], 60)
console.log(await table.indexStats(indexName))
// {
//   numIndexedRows: 1000,
//   numUnindexedRows: 0,
//   indexType: 'IVF_PQ',
//   distanceType: 'cosine'
// }
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