CREATE VECTOR INDEX

Introduced or updated: v1.2.777

Creates a Vector index on a VECTOR column for a table to enable efficient similarity search using the HNSW (Hierarchical Navigable Small World) algorithm.

Syntax

-- Create a Vector index on an existing table
CREATE [OR REPLACE] VECTOR INDEX [IF NOT EXISTS] <index_name>
ON [<database>.]<table_name>(<column>)
distance = '<metric>' [m = <number>] [ef_construct = <number>]

-- Create a Vector index when creating a table
CREATE [OR REPLACE] TABLE <table_name> (
    <column_definitions>,
    VECTOR INDEX <index_name> (<column>)
        distance = '<metric>' [m = <number>] [ef_construct = <number>]
)...

Parameters

• distance (required) - Specifies the distance metric(s) to use for similarity search. Multiple metrics can be combined with commas:

  • 'cosine' - Cosine distance (best for semantic similarity, text embeddings)
  • 'l1' - L1 distance / Manhattan distance (good for feature comparison, sparse data)
  • 'l2' - L2 distance / Euclidean distance (best for geometric similarity, image features)
  • Example: distance = 'cosine,l1,l2' supports all three metrics

• m (optional, default: 16) - Controls the number of bidirectional connections each node has in the HNSW graph:

  • Higher values increase memory usage but can improve search accuracy
  • Must be greater than 0
  • Typical range: 8-64

• ef_construct (optional, default: 100) - Controls the size of the dynamic candidate list during index construction:

  • Higher values improve index quality but increase construction time and memory
  • Must be >= 40
  • Typical range: 40-500

How Vector Index Works

Vector indexes in Databend use the HNSW algorithm to build a multi-layered graph structure:

1. Graph Structure: Each vector is a node with connections to its nearest neighbors
2. Search Process: Queries navigate through graph layers, from coarse to fine, to find approximate nearest neighbors quickly
3. Quantization: Raw vectors are quantized to reduce storage and improve query performance (with negligible accuracy loss)
4. Automatic Building: The index is automatically built as data is written. Every INSERT, COPY, or data load operation automatically generates the index for new rows - no manual maintenance required

Examples

Creating a Table with Vector Index

-- Simple vector index for embeddings
CREATE TABLE documents (
    id INT,
    title VARCHAR,
    content TEXT,
    embedding VECTOR(1024),
    VECTOR INDEX idx_embedding(embedding) distance = 'cosine'
);

Creating a Vector Index with Custom Parameters

-- Vector index with multiple distance metrics and tuned parameters
CREATE TABLE images (
    id INT,
    filename VARCHAR,
    feature_vector VECTOR(512),
    VECTOR INDEX idx_features(feature_vector)
        distance = 'cosine,l2'
        m = 32
        ef_construct = 200
);

Creating a Vector Index on an Existing Table

CREATE TABLE products (
    id INT,
    name VARCHAR,
    description TEXT,
    embedding VECTOR(768)
);

-- Add vector index after table creation
CREATE VECTOR INDEX idx_product_embedding
ON products(embedding)
distance = 'cosine,l1,l2'
m = 20
ef_construct = 150;

Multiple Vector Indexes on Different Columns

CREATE TABLE multimodal_data (
    id INT,
    text_embedding VECTOR(384),
    image_embedding VECTOR(512),
    VECTOR INDEX idx_text(text_embedding) distance = 'cosine',
    VECTOR INDEX idx_image(image_embedding) distance = 'l2'
);

Viewing Indexes

Use SHOW INDEXES to view all indexes:

SHOW INDEXES;

Result:

┌──────────────────────┬────────┬──────────┬────────────────────────────┬──────────────────────────┐
│ name                 │ type   │ original │ definition                 │ created_on               │
├──────────────────────┼────────┼──────────┼────────────────────────────┼──────────────────────────┤
│ idx_embedding        │ VECTOR │          │ documents(embedding)       │ 2025-05-13 01:22:34.123  │
│ idx_product_embedding│ VECTOR │          │ products(embedding)        │ 2025-05-13 01:23:45.678  │
└──────────────────────┴────────┴──────────┴────────────────────────────┴──────────────────────────┘

Using Vector Index for Similarity Search

-- Create a table with vector index
CREATE TABLE wiki_articles (
    id INT,
    title VARCHAR,
    embedding VECTOR(8),
    VECTOR INDEX idx_embedding(embedding) distance = 'cosine'
);

-- Insert sample data (8-dimensional vectors for demonstration)
INSERT INTO wiki_articles VALUES
(1, 'Machine Learning', [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8]),
(2, 'Deep Learning', [0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, 0.85]),
(3, 'Natural Language Processing', [0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]),
(4, 'Computer Vision', [0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1]);

-- Find the 2 most similar articles to a query vector using cosine distance
SELECT id, title, cosine_distance(embedding, [0.12, 0.22, 0.32, 0.42, 0.52, 0.62, 0.72, 0.82]) AS distance
FROM wiki_articles
ORDER BY distance ASC
LIMIT 2;

Result:

┌────┬─────────────────┬──────────────┐
│ id │ title           │ distance     │
├────┼─────────────────┼──────────────┤
│  1 │ Machine Learning│ 0.00012345   │
│  2 │ Deep Learning   │ 0.00023456   │
└────┴─────────────────┴──────────────┘

Performance Tuning

Choosing Distance Metrics

Choose the appropriate distance metric based on your use case. See Vector Functions for querying with distance functions.

• Cosine distance: Best for text embeddings from models like BERT, GPT, where vector magnitude doesn't matter
• L2 (Euclidean) distance: Best for image features, spatial data where absolute differences matter
• L1 (Manhattan) distance: Good for sparse vectors and when you want to emphasize individual dimension differences

Tuning HNSW Parameters

Parameter	Lower Value	Higher Value
m	Less memory, faster construction	Better accuracy, more memory
ef_construct	Faster construction, lower quality	Better quality, slower construction

Recommended configurations:

• Small datasets (< 100K vectors): Default settings (m=16, ef_construct=100)
• Medium datasets (100K - 1M vectors): m=24, ef_construct=150
• Large datasets (> 1M vectors): m=32, ef_construct=200
• High accuracy requirements: m=48, ef_construct=300

Limitations

• Vector indexes only support columns with VECTOR data type
• The distance parameter is required; indexes without it will be ignored
• Quantization may introduce negligible errors in distance calculations (typically < 0.01%)
• Index size increases with higher m values (approximately m * vector_dimension * 4 bytes per vector)