PostgreSQL Semantic Search

PostgreSQL Semantic Search

Quick Start

1. Setup

CREATE EXTENSION IF NOT EXISTS vector;

CREATE TABLE documents (
    id SERIAL PRIMARY KEY,
    content TEXT NOT NULL,
    embedding vector(1536)  -- 1536-dim embedding
    -- Or: embedding halfvec(3072)  -- 3072-dim embedding (halfvec = 50% memory)
);

2. Basic Semantic Search

SELECT id, content, 1 - (embedding <=> query_vec) AS similarity
FROM documents
ORDER BY embedding <=> query_vec
LIMIT 10;

3. Add Index (> 10k documents)

CREATE INDEX ON documents USING hnsw (embedding vector_cosine_ops);

Docker Quick Start

# pgvector with PostgreSQL 17
docker run -d --name pgvector-db \
  -e POSTGRES_PASSWORD=postgres \
  -p 5432:5432 \
  pgvector/pgvector:pg17

# Or PostgreSQL 18 (latest)
docker run -d --name pgvector-db \
  -e POSTGRES_PASSWORD=postgres \
  -p 5432:5432 \
  pgvector/pgvector:pg18

# ParadeDB (includes pgvector + pg_search + BM25)
docker run -d --name paradedb \
  -e POSTGRES_PASSWORD=postgres \
  -p 5432:5432 \
  paradedb/paradedb:latest

Connect: psql postgresql://postgres:postgres@localhost:5432/postgres

Cheat Sheet

Distance Operators

embedding <=> query  -- Cosine distance (1 - similarity)
embedding <-> query  -- L2/Euclidean distance
embedding <#> query  -- Negative inner product

Common Queries

-- Top 10 similar (cosine)
SELECT * FROM docs ORDER BY embedding <=> $1 LIMIT 10;

-- With similarity score
SELECT *, 1 - (embedding <=> $1) AS similarity FROM docs ORDER BY 2 DESC LIMIT 10;

-- With threshold
SELECT * FROM docs WHERE embedding <=> $1 < 0.3 ORDER BY 1 LIMIT 10;

-- Preload index (run on startup)
SELECT 1 FROM docs ORDER BY embedding <=> $1 LIMIT 1;

Index Quick Reference

-- HNSW (recommended)
CREATE INDEX ON docs USING hnsw (embedding vector_cosine_ops);

-- With tuning
CREATE INDEX ON docs USING hnsw (embedding vector_cosine_ops)
WITH (m = 24, ef_construction = 200);

-- Query-time recall
SET hnsw.ef_search = 100;

-- Iterative scan for filtered queries (pgvector 0.8+)
SET hnsw.iterative_scan = relaxed_order;
SET ivfflat.iterative_scan = on;

Decision Trees

Choose Search Method

Query type?
├─ Conceptual/meaning-based → Pure vector search
├─ Exact terms/names → Pure keyword search (FTS)
├─ Fuzzy/typo-tolerant → pg_trgm trigram similarity
├─ Autocomplete/prefix → pg_trgm + prefix index
├─ Substring (LIKE/ILIKE) → pg_trgm GIN index
└─ Mixed/unknown → Hybrid search
    ├─ Simple setup → FTS + RRF (no extra extensions)
    ├─ Better ranking → BM25 + RRF (pg_search extension)
    └─ Full-featured → ParadeDB (Elasticsearch alternative)

Choose Index Type

Document count?
├─ < 10,000 → No index needed
├─ 10k - 1M → HNSW (best recall)
└─ > 1M → IVFFlat (less memory) or HNSW

Choose Vector Type

Choose by dimensions, not by provider — the column type only depends on embedding size and pgvector's HNSW index limits.

Embedding dimensions (N)?
├─ N ≤ 2000  → vector(N)   — HNSW indexable directly
├─ 2000 < N ≤ 4000 → halfvec(N) — vector(N)'s HNSW limit is 2000; halfvec extends to 4000
└─ N > 4000  → vector(N) without HNSW, or quantize via dimensionality reduction

Common embedding dimensions are 1536 and 3072, but sizes vary by provider and model — check the provider's docs for the embedding you're using.

For multilingual / non-English content, prefer multilingual-tuned embedding models (look for "multilingual" in the model name). Models tuned only on English may handle compound words and inflection poorly.

Storage vs. index trick for 2000 < N ≤ 4000: keep the column as vector(N) (full float4, useful for future re-embedding or re-ranking experiments) and only cast at index creation and query time. This preserves precision on disk while staying within HNSW's dimension limit.

CREATE INDEX ON docs USING hnsw ((embedding::halfvec(3072)) halfvec_cosine_ops);
-- Query must cast identically so the planner picks the index:
SELECT * FROM docs ORDER BY embedding::halfvec(3072) <=> $1 LIMIT 10;

If storage is tight or you never plan to re-embed, use halfvec(N) as the column type directly.

Measure before adopting

Every optimization in this skill (hybrid fusion, reranking, query expansion, embedding-model swaps) can regress on a specific corpus. Vendor and paper benchmarks are usually English, general-domain. Real counter-examples observed in production:

• Query expansion (HyDE) regressing Hit@5 by tens of points on a domain corpus.
• A widely recommended reranker regressing Hit@5 double-digits on multilingual text.

Rule: build a domain eval set (evaluation.md), then A/B each change. Adopt with ≥ +3 pp Hit@5 and p95 latency within budget; reject otherwise.

Operators

Operator	Distance	Use Case
<=>	Cosine	Text embeddings (default)
<->	L2/Euclidean	Image embeddings
<#>	Inner product	Normalized vectors

SQL Functions

Semantic Search

• match_documents(query_vec, threshold, limit) - Basic search
• match_documents_filtered(query_vec, metadata_filter, threshold, limit) - With JSONB filter
• match_chunks(query_vec, threshold, limit) - Search document chunks

Fuzzy Search (pg_trgm)

• fuzzy_search_trigram(query_text, threshold, limit) - Trigram similarity search
• autocomplete_search(prefix, limit) - Prefix + fuzzy autocomplete
• hybrid_search_fuzzy_semantic(query_text, query_vec, limit, rrf_k) - Fuzzy + vector RRF
• weighted_fts_search(query_text, language, limit) - FTS with title/content weighting

Hybrid Search (FTS)

• hybrid_search_fts(query_vec, query_text, limit, rrf_k, language) - FTS + RRF
• hybrid_search_weighted(query_vec, query_text, limit, sem_weight, kw_weight) - Linear combination
• hybrid_search_fallback(query_vec, query_text, limit) - Graceful degradation

Hybrid Search (BM25)

• hybrid_search_bm25(query_vec, query_text, limit, rrf_k) - BM25 + RRF
• hybrid_search_bm25_highlighted(...) - With snippet highlighting
• hybrid_search_chunks_bm25(...) - For RAG with chunks

Re-ranking (Optional)

Two-stage retrieval improves precision: fast recall → precise rerank with a cross-encoder. Use when results need higher precision and you have <50 candidates after initial retrieval.

Key rule: rerankers must be wrapped so a failure (missing key, HTTP error, timeout) returns null and the caller falls back to original retrieval order — never let a reranker outage break search.

For provider comparison, generic Promise<T | null> wrapper, and self-hosted options, see reranking.md.

Multilingual / non-English content tips

When the corpus is non-English (Finnish, German, French, Spanish, etc.):

• FTS language config: pass the matching language to to_tsvector(language, text) to apply the built-in snowball stemmer (e.g., 'finnish' handles opiskelija → opiskelij). For mixed-language corpora, use 'simple' and rely on prefix/trigram fallbacks instead.

• Combine stemmer + unaccent for accent-insensitive matching ("café" matches "cafe"). See hybrid-search.md → Custom FTS configuration for the 3-step DDL pattern.

• Prefix tsquery for languages with rich inflection (no full morphology engine required):

  CREATE OR REPLACE FUNCTION prefix_tsquery(p text)
  RETURNS tsquery LANGUAGE sql IMMUTABLE AS $$
    SELECT to_tsquery('simple',
      string_agg(word || ':*', ' & '))
    FROM regexp_split_to_table(lower(regexp_replace(p, '[^\w\s-]', ' ', 'g')), '\s+') AS word
    WHERE length(word) >= 2
  $$;
  

  Matches kartta, karttaa, karttoja from a single kartta:* token.

• Compound-word fallback: pair semantic search with pg_trgm similarity to catch compound-word misses (e.g., a query for "ammattikorkea" should still find "ammattikorkeakoulu").

• BM25 stemmer in ParadeDB: tokenize with { "type": "default", "stemmer": "<language>" } — a raw tokenizer only matches full fields.

• Multilingual embeddings: prefer models explicitly trained on your target language(s). English-only embeddings often miss inflected forms and compound words. The gap can be large — multilingual-tuned embeddings have been observed to beat general-purpose English-tuned ones by 10+pp Hit@5 on non-English retrieval. Benchmark your specific language + domain before committing.

References

• fuzzy-search.md - pg_trgm, fuzzy matching, LIKE/ILIKE, autocomplete, advanced FTS
• paradedb.md - ParadeDB full-text search (Elasticsearch alternative)
• vector-types.md - vector vs halfvec, dimensions, storage
• indexing.md - HNSW, IVFFlat, GIN parameters
• hybrid-search.md - FTS, BM25, RRF algorithms
• performance.md - Cold-start, memory, HNSW vs IVFFlat
• evaluation.md - Eval-set construction, Hit@K / MRR, adoption thresholds, reranker/expansion benchmarking
• reranking.md - Two-stage retrieval, graceful fallback, when rerankers regress

Scripts

• setup.sql - Extension and table setup
• semantic_search.sql - Semantic search functions
• hybrid_search_fts.sql - FTS hybrid functions
• hybrid_search_bm25.sql - BM25 hybrid functions
• fuzzy_search.sql - pg_trgm fuzzy search, autocomplete, weighted FTS
• indexes.sql - Index creation scripts

Common Patterns

TypeScript Integration (Supabase)

// Semantic search
const { data } = await supabase.rpc('match_documents', {
  query_embedding: embedding,
  match_threshold: 0.7,
  match_count: 10
});

// Hybrid search
const { data } = await supabase.rpc('hybrid_search_fts', {
  query_embedding: embedding,
  query_text: userQuery,
  match_count: 10,
  rrf_k: 60,
  fts_language: 'simple'
});

Drizzle ORM

import { sql } from 'drizzle-orm';

const results = await db.execute(sql`
  SELECT * FROM match_documents(
    ${embedding}::vector(1536),
    0.7,
    10
  )
`);

Troubleshooting

Symptom	Cause	Solution
Index not used	< 10k rows or planner choice	Normal for small tables, check with EXPLAIN
Slow first query (30-60s)	HNSW cold-start	SELECT pg_prewarm('idx_name') or preload query
Poor recall	Low ef_search	SET hnsw.ef_search = 100 or higher
FTS returns nothing	Wrong language config	Use 'simple' for mixed/unknown languages
Memory error on index build	maintenance_work_mem too low	Increase to 2GB+
Cosine similarity > 1	Vectors not normalized	Normalize before insert or use L2
Slow inserts	Index overhead	Batch inserts, consider IVFFlat
Fuzzy search slow	Missing trigram index	CREATE INDEX USING gin (col gin_trgm_ops)
ILIKE '%x%' slow	No pg_trgm GIN index	Enable pg_trgm + create GIN trigram index
% operator error	pg_trgm not installed	CREATE EXTENSION IF NOT EXISTS pg_trgm

Compatibility

• pgvector: 0.8+ recommended (iterative scans, halfvec). Check pgvector releases.
• pg_search: Check ParadeDB releases for latest.
• PostgreSQL: 17+ recommended. pgvector supports 13-18.

Related Skills

Need	Skill
General Postgres performance, indexes, RLS, connection pooling	/supabase-postgres-best-practices
Chatbot orchestration, session DB, tool calls, HITL, feedback	/nextjs-chatbot
AI SDK v6 usage for embeddings and retrieval	/ai-sdk-6

For ParadeDB-specific questions, always apply the Documentation Fetch Policy in references/paradedb.md — live docs at https://docs.paradedb.com/llms-full.txt are the authoritative source.

External Documentation

Core

• pgvector GitHub - Official extension, latest features
• PostgreSQL FTS - Built-in full-text search

Embedding providers

• OpenAI Embeddings - model list + dimensions
• Voyage Embeddings - includes multilingual model
• Cohere Embed - model list
• HuggingFace Hub - open-weight embeddings

Reranker providers

• Cohere Rerank
• Voyage Rerank
• Zerank
• Sentence Transformers - self-hosted cross-encoders

Hosting / extensions

• Supabase Vector Guide - Supabase-specific integration
• ParadeDB pg_search - BM25 extension documentation
• ParadeDB AI Docs - Fetch for latest ParadeDB API (always current)