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Database / pgvector basics Interview Questions

What data types does pgvector provide and how do you define vector columns?

pgvector introduces several new data types to PostgreSQL for storing vector data. The primary type is vector, with additional types for half-precision and binary vectors added in later releases.

pgvector data types
TypeStoragePrecisionMax dimensionsUse case
vector(n)4 bytes per dimension32-bit float (single precision)16,000Standard embeddings (OpenAI, Cohere, etc.)
halfvec(n)2 bytes per dimension16-bit float (half precision)16,000Reduced storage; slight precision trade-off
bit(n)~1 bit per dimensionBinary (0 or 1)64,000Binary quantised embeddings; very compact
sparsevec(n)Only non-zero values stored32-bit float1,000,000Sparse vectors (most dimensions are 0)
-- Basic vector column (fixed dimensions)
CREATE TABLE documents (
    id         BIGSERIAL PRIMARY KEY,
    content    TEXT,
    embedding  VECTOR(1536)    -- 1536-dim for text-embedding-3-small
);

-- Half-precision to save storage (2x space reduction)
CREATE TABLE documents_small (
    id         BIGSERIAL PRIMARY KEY,
    embedding  HALFVEC(1536)
);

-- Binary vectors (very compact; special distance operators)
CREATE TABLE binary_items (
    id         BIGSERIAL PRIMARY KEY,
    embedding  BIT(1536)
);

-- Variable dimensions (no dimension constraint)
CREATE TABLE flexible (
    id         BIGSERIAL PRIMARY KEY,
    embedding  VECTOR         -- accepts any dimension
);
-- NOTE: indexes on variable-dim columns require expression/partial indexes

How many bytes does each dimension occupy in a standard vector(n) column?
What is the primary advantage of using halfvec(n) instead of vector(n)?

More Related questions...

What is pgvector and what problem does it solve for developers? What are vector embeddings and why are they central to pgvector's use? How do you install and enable pgvector in PostgreSQL? What data types does pgvector provide and how do you define vector columns? What distance operators does pgvector provide and when do you use each? How do you insert and update vector data in pgvector? How do you perform a basic nearest-neighbour search with pgvector? What is the difference between exact and approximate nearest-neighbour search in pgvector? What is the HNSW index in pgvector and how do you create and tune it? What is the IVFFlat index in pgvector and how does it compare to HNSW? How do you use pgvector with Python and psycopg2? How do you use pgvector with SQLAlchemy and Python ORMs? How do you combine vector similarity search with SQL filters (hybrid search) in pgvector? How do you bulk-load vectors efficiently into pgvector? What operator classes must you use when creating pgvector indexes and why do they matter? How does pgvector fit into a RAG (Retrieval-Augmented Generation) pipeline? What is cosine distance vs cosine similarity and which does pgvector return? How do you check and monitor pgvector index creation progress? What is the pgvector maximum supported dimensions limit and how do you handle high-dimensional vectors? How do you use pgvector with LangChain for building AI applications? How does pgvector handle NULL values in vector columns? What are partial indexes in pgvector and when should you use them? How do you perform vector arithmetic and other vector functions in pgvector? How does pgvector compare to dedicated vector databases like Pinecone, Weaviate, and Qdrant? What is the difference between L1 and L2 distance in pgvector? How do you store and query vector embeddings with pgvector in a real schema design? What is halfvec and when should you use it to reduce storage costs? How do you handle vector dimensionality mismatches in pgvector? How do you use pgvector with Django? What are common performance tuning techniques for pgvector at scale? How do you implement semantic search with pgvector and a similarity threshold? How do you use pgvector with asyncpg or asyncio in Python? What is vector quantisation and how does pgvector support binary quantisation? How does pgvector integrate with managed PostgreSQL services? How do you use the inner product operator <#> with pgvector and when is it appropriate? How do you combine pgvector with full-text search (hybrid keyword + semantic search)? What PostgreSQL configuration parameters affect pgvector performance? How do you implement recommendation systems using pgvector? How do you use EXPLAIN and EXPLAIN ANALYZE to debug pgvector queries? What are best practices for a production pgvector deployment?
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