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Database / pgvector basics Interview Questions

What is the difference between exact and approximate nearest-neighbour search in pgvector?

pgvector supports two search modes with very different performance and accuracy characteristics. Choosing the right one depends on dataset size and whether you need perfect recall.

Exact vs Approximate search
AspectExact (sequential scan)Approximate (ANN with index)
MethodComputes distance to EVERY rowSearches a smart subset of rows
Recall100% - always finds the true nearest neighboursLess than 100% - may miss some true neighbours
Speed at scaleSlow O(n) per querySub-linear; fast even at millions of rows
ConfigurationNo index neededRequires HNSW or IVFFlat index
Good for< ~100k rows or when 100% recall neededMillions of rows; speed more important than perfect recall
Index requiredNoYes (HNSW or IVFFlat)
-- EXACT search (no index): scans every row
-- Accurate but slow at scale
SELECT id, embedding <-> '[3,1,2]' AS dist
FROM items
ORDER BY dist
LIMIT 5;

-- APPROXIMATE search (with HNSW index): fast, high recall
-- Create the index:
CREATE INDEX ON items USING hnsw (embedding vector_l2_ops);

-- Same query syntax - PostgreSQL uses the index automatically:
SELECT id, embedding <-> '[3,1,2]' AS dist
FROM items
ORDER BY dist
LIMIT 5;
-- Now uses HNSW index for fast approximate search

-- Check which index was used:
EXPLAIN SELECT id FROM items ORDER BY embedding <-> '[1,2,3]' LIMIT 5;
-- Look for 'Index Scan using items_embedding_idx' in the plan

-- Force exact search even when index exists:
SET enable_indexscan = off;
SELECT id FROM items ORDER BY embedding <-> '[1,2,3]' LIMIT 5;
RESET enable_indexscan;

At what approximate dataset size does the pgvector documentation suggest considering an ANN index instead of exact search?
What is 'recall' in the context of approximate nearest-neighbour search?

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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