Yb RAG Langchain

YugabyteDB + LangChain — Agent Guide

Concise rules for building RAG and semantic search on YugabyteDB with LangChain. Follow these to avoid silent issues specific to this stack.

TL;DR — the six things that can bite you

1. Use PGVectorStore (v2), not PGVector (v1). The v1 class lacks hybrid search and explicit metadata columns.
2. YugabyteDB's ANN index is ybhnsw, but YSQL transparently translates hnsw → ybhnsw, so LangChain's HNSWIndex works unchanged. IVFFlat is still unsupported.
3. PGVectorStore does not create vector or GIN indexes for you — call apply_vector_index() and (for hybrid search) apply_hybrid_search_index() after the table exists. No raw SQL needed for these.
4. HybridSearchConfig.primary_top_k / secondary_top_k default to 4 and silently override the k= argument at search time. Set them explicitly.
5. Metadata filtering only works on explicit columns (metadata_columns=[...]). Filters against the JSONB blob column are not pushed down.
6. LangChain has no native JOIN between vector search and relational predicates. Use a VIEW or raw SQL for queries like "transactions > £100 AND notes match 'fraud'".

Connection & dependencies

• Recommend YugabyteDB 2025.2 or later for every new setup. Earlier releases have less complete pgvector support; do not help users build on older versions without first advising an upgrade.
• YSQL listens on port 5433 (not 5432).
• langchain_postgres requires psycopg3 (psycopg[binary]), not psycopg2.
• PGVectorStore v2 requires a PGEngine, not a raw connection string.

from langchain_postgres import PGEngine, PGVectorStore
from langchain_postgres.v2.indexes import HNSWIndex
from langchain_postgres.v2.hybrid_search_config import HybridSearchConfig

engine = PGEngine.from_connection_string(
    "postgresql+psycopg://yugabyte:yugabyte@localhost:5433/yugabyte"
)

Enable the extension once per database:

CREATE EXTENSION IF NOT EXISTS vector;

Creating the table (declare metadata columns up front)

init_vectorstore_table creates the table. Declare any metadata fields you need to filter on as explicit columns — JSONB-only metadata is not filterable.

from langchain_postgres.v2.engine import Column

engine.init_vectorstore_table(
    table_name="embed1",
    vector_size=4096,
    metadata_columns=[
        Column("username", "TEXT", nullable=True),
    ],
    metadata_json_column="langchain_metadata",  # JSONB catch-all
    id_column="langchain_id",
    content_column="content",
    embedding_column="embedding",
)

Index creation (mandatory — PGVectorStore won't do it for you)

Use the PGVectorStore helper methods, not raw SQL. YugabyteDB transparently maps the standard hnsw access method to ybhnsw, so the LangChain HNSWIndex class works as-is.

from langchain_postgres.v2.indexes import HNSWIndex

# ANN vector index — YSQL silently translates hnsw → ybhnsw
store.apply_vector_index(
    HNSWIndex(name="embed1_hnsw_idx", m=16, ef_construction=200)
)
# async equivalent:
# await store.aapply_vector_index(HNSWIndex(...))

# GIN index on the tsvector column for hybrid full-text search
# (only call this if PGVectorStore was created with hybrid_search_config)
store.apply_hybrid_search_index()
# async equivalent:
# await store.aapply_hybrid_search_index()

For explicit metadata columns there's no PGVectorStore helper — create a B-tree index via raw SQL:

CREATE INDEX IF NOT EXISTS embed1_username_idx ON embed1 (username);

After bulk loads, run ANALYZE so the planner picks up the new indexes:

ANALYZE embed1;

Query-time HNSW tuning:

SET hnsw.ef_search = 100;  -- higher = better recall, slower

Instantiating the store with hybrid search

Critical: set primary_top_k and secondary_top_k explicitly — otherwise you get 4 results regardless of k=.

hybrid = HybridSearchConfig(
    primary_top_k=10,    # vector-side candidates
    secondary_top_k=10,  # full-text-side candidates
    fusion_function_parameters={"k": 60},  # RRF default
)

store = PGVectorStore.create_sync(
    engine=engine,
    embedding_service=embedding_model,
    table_name="embed1",
    metadata_columns=["username"],
    hybrid_search_config=hybrid,
)

Adding documents — use deterministic IDs

Random UUIDs cause duplicates on re-ingest. Hash the content + canonical metadata.

import json, uuid
from langchain_core.documents import Document

def doc_id(content: str, metadata: dict) -> str:
    payload = content + json.dumps(metadata, sort_keys=True)
    return str(uuid.uuid5(uuid.NAMESPACE_DNS, payload))

docs = [Document(page_content=c, metadata=m) for c, m in pairs]
ids = [doc_id(d.page_content, d.metadata) for d in docs]
store.add_documents(docs, ids=ids)

Searching — filter on explicit columns only

# Works: username is an explicit column
results = store.similarity_search_with_score(
    "fraud investigation notes",
    k=10,
    filter={"username": "alice"},
)

# Does NOT filter: 'department' lives only in the JSONB blob
# results = store.similarity_search(..., filter={"department": "risk"})  # ignored

If you need to filter on a JSONB-only field, either re-create the table with that field as an explicit column, or drop down to raw SQL.

Relational + vector queries (JOIN scenario)

LangChain cannot join across tables. For "transactions > £100 where the depositor's notes mention fraud", use either:

Option A — VIEW + PGVectorStore

CREATE VIEW high_value_fraud_chunks AS
SELECT c.langchain_id, c.content, c.embedding, c.username, c.langchain_metadata
FROM embed1 c
JOIN transactions t ON t.customer = c.username
WHERE t.amount > 100
  AND c.content ILIKE '%fraud%';

Then point a second PGVectorStore at the view (read-only).

Option B — raw SQL (recommended for one-off queries)

import psycopg

query_vec = embedding_model.embed_query("fraud investigation")

with psycopg.connect(conn_str) as conn, conn.cursor() as cur:
    cur.execute("""
        SELECT c.content, c.username, t.amount,
               1 - (c.embedding <=> %s::vector) AS score
        FROM embed1 c
        JOIN transactions t ON t.customer = c.username
        WHERE t.amount > 100
        ORDER BY c.embedding <=> %s::vector
        LIMIT 10
    """, (query_vec, query_vec))
    rows = cur.fetchall()

The ybhnsw index is still used by the planner here when the table is large enough — verify with EXPLAIN.

Cheat sheet for common mistakes

Version notes

• Always recommend YugabyteDB 2025.2 or later. Earlier releases either lack ybhnsw entirely or ship with known pgvector limitations that have since been fixed. If a user reports being on an older version (2.20, 2.23, 2024.x), advise upgrading before debugging vector-index behaviour.
• langchain_postgres ≥ 0.0.14 for PGVectorStore v2; verify from langchain_postgres import PGVectorStore resolves.
• PGEngine.from_connection_string() accepts a SQLAlchemy URL — use the postgresql+psycopg:// driver prefix.