2nd/10_Wiki/Topics/AI_and_ML/Semantic Search.md

---
id: wiki-2026-0508-semantic-search
title: Semantic Search
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Vector Search, Dense Retrieval, Neural Search, Semantic Search with AI]
duplicate_of: none
source_trust_level: A
confidence_score: 0.93
verification_status: applied
tags: [search, retrieval, embeddings, vector-db, rag]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: faiss
---

# Semantic Search

## 매 한 줄
> **"매 query → embedding → ANN nearest neighbors in vector space"**. 매 BM25 매 lexical 한계를 dense retrieval (DPR, ColBERT) 매 극복. 매 2026 production: hybrid (BM25 + dense + reranker), 매 모범: OpenAI text-embedding-3-large, Cohere v3, Voyage-3, BGE-M3, Jina-v3.

## 매 핵심

### 매 Pipeline
1. **Index time**: doc → chunk → embed → vector DB.
2. **Query time**: query → embed → ANN search → (rerank) → results.
3. **Hybrid**: BM25 score + dense score → RRF or weighted.
4. **Rerank**: cross-encoder on top-100 → top-10 (Cohere Rerank, BGE-Reranker).

### 매 Embedding models (2026)
- **OpenAI text-embedding-3-large** (3072d, MRL truncatable).
- **Cohere embed-v3** (multilingual, dot-product).
- **Voyage-3** (state-of-art retrieval).
- **BGE-M3** (open, multi-vector, sparse+dense).
- **Jina-v3** (8k context, MRL).
- **NV-Embed-v2** (NVIDIA, MTEB top).

### 매 ANN algorithms
- **HNSW** (graph): 매 default, fast, high recall.
- **IVF-PQ** (Faiss): 매 huge scale, compressed.
- **DiskANN**: 매 on-disk billion-scale.
- **ScaNN** (Google): 매 best at fixed memory.

### 매 Vector DBs
- **Pinecone** (managed).
- **Weaviate** (open + hybrid built-in).
- **Qdrant** (Rust, fast).
- **Milvus** (large-scale).
- **pgvector** (Postgres).
- **LanceDB** (embedded, columnar).
- **Turbopuffer** (serverless 2024+).

### 매 응용
1. RAG knowledge retrieval.
2. Code search (Cursor, Sourcegraph).
3. E-commerce / product search.
4. Multimodal (CLIP image+text).

## 💻 패턴

### Basic dense retrieval
```python
from openai import OpenAI
import numpy as np
import faiss

client = OpenAI()

def embed(texts):
    r = client.embeddings.create(model="text-embedding-3-large", input=texts)
    return np.array([d.embedding for d in r.data], dtype="float32")

docs = ["Doc 1 text...", "Doc 2 text...", "..."]
doc_vecs = embed(docs)
index = faiss.IndexHNSWFlat(3072, 32)
faiss.normalize_L2(doc_vecs)
index.add(doc_vecs)

q_vec = embed(["What is X?"])
faiss.normalize_L2(q_vec)
D, I = index.search(q_vec, 10)
print([docs[i] for i in I[0]])
```

### Hybrid (BM25 + dense) with RRF
```python
from rank_bm25 import BM25Okapi

bm25 = BM25Okapi([d.split() for d in docs])

def rrf(rankings, k=60):
    scores = {}
    for ranking in rankings:
        for rank, doc_id in enumerate(ranking):
            scores[doc_id] = scores.get(doc_id, 0) + 1 / (k + rank)
    return sorted(scores.items(), key=lambda x: -x[1])

def hybrid_search(query, k=10):
    bm25_top = np.argsort(-bm25.get_scores(query.split()))[:50]
    q_vec = embed([query]); faiss.normalize_L2(q_vec)
    _, dense_top = index.search(q_vec, 50)
    fused = rrf([bm25_top.tolist(), dense_top[0].tolist()])
    return [docs[i] for i, _ in fused[:k]]
```

### Cross-encoder reranking
```python
import cohere
co = cohere.Client()

def rerank(query, candidates, top_n=10):
    r = co.rerank(query=query, documents=candidates,
                  model="rerank-english-v3.0", top_n=top_n)
    return [candidates[res.index] for res in r.results]
```

### Chunking with overlap
```python
def chunk_text(text, size=500, overlap=50):
    words = text.split()
    chunks = []
    for i in range(0, len(words), size - overlap):
        chunk = " ".join(words[i:i+size])
        chunks.append(chunk)
    return chunks

# 매 better: 매 semantic chunker (매 paragraph + heading aware)
from langchain.text_splitter import RecursiveCharacterTextSplitter
splitter = RecursiveCharacterTextSplitter(chunk_size=500, chunk_overlap=50,
                                          separators=["\n\n", "\n", ". ", " "])
```

### MRL truncation (Matryoshka)
```python
# text-embedding-3-large: 3072d, truncatable to 256/512/1024
def embed_mrl(text, dim=512):
    full = embed([text])[0]
    truncated = full[:dim]
    return truncated / np.linalg.norm(truncated)
# 매 6× memory savings, 매 ~95% recall.
```

### ColBERT (multi-vector late interaction)
```python
from colbert.modeling.colbert import ColBERT

# 매 token-level vectors per query+doc; 매 max-sim per query token then sum.
def colbert_score(query_vecs, doc_vecs):
    # query_vecs: [Q, d], doc_vecs: [D, d]
    sim = query_vecs @ doc_vecs.T  # [Q, D]
    return sim.max(axis=1).sum()   # 매 sum of per-token max
```

### pgvector hybrid (production)
```sql
CREATE TABLE docs (id bigserial, content text, embedding vector(1536),
                   tsv tsvector GENERATED ALWAYS AS (to_tsvector('english', content)) STORED);
CREATE INDEX ON docs USING hnsw (embedding vector_cosine_ops);
CREATE INDEX ON docs USING gin (tsv);

-- Hybrid query
WITH dense AS (
  SELECT id, 1 - (embedding <=> $1) AS score FROM docs ORDER BY embedding <=> $1 LIMIT 50
), sparse AS (
  SELECT id, ts_rank_cd(tsv, websearch_to_tsquery($2)) AS score
  FROM docs WHERE tsv @@ websearch_to_tsquery($2) LIMIT 50
)
SELECT id, COALESCE(d.score, 0) * 0.7 + COALESCE(s.score, 0) * 0.3 AS score
FROM dense d FULL OUTER JOIN sparse s USING (id)
ORDER BY score DESC LIMIT 10;
```

### Multimodal CLIP search
```python
import torch
from transformers import CLIPModel, CLIPProcessor

model = CLIPModel.from_pretrained("openai/clip-vit-large-patch14")
proc = CLIPProcessor.from_pretrained("openai/clip-vit-large-patch14")

def embed_image(img):
    with torch.no_grad():
        return model.get_image_features(**proc(images=img, return_tensors="pt"))

def embed_text(t):
    with torch.no_grad():
        return model.get_text_features(**proc(text=t, return_tensors="pt"))
# 매 same vector space → cross-modal search.
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Quick prototype | 매 OpenAI embeddings + Faiss/LanceDB |
| Production RAG | 매 hybrid (BM25 + dense) + Cohere rerank |
| Self-host open | 매 BGE-M3 + Qdrant + BGE-reranker |
| Multilingual | 매 BGE-M3, Cohere multilingual, embed-v4 |
| Code search | 매 Voyage-code-3 또는 jina-code-v2 |
| Multimodal | 매 CLIP / SigLIP / Jina-CLIP |

**기본값**: 매 production RAG → hybrid (BM25 + dense) + cross-encoder rerank.

## 🔗 Graph
- 부모: [[Information Retrieval]] · [[Embeddings]]
- 변형: [[Dense Retrieval]] · [[Sparse Retrieval]] · [[Information-Retrieval-IR|Hybrid Search]] · [[ColBERT]]
- 응용: [[RAG]] · [[Recommender Systems]]
- Adjacent: [[BM25]] · [[Cross-Encoder Reranking]] · [[CLIP]]

## 🤖 LLM 활용
**언제**: 매 RAG retrieval, 매 semantic deduplication, 매 cross-lingual search, 매 recommendation.
**언제 X**: 매 exact-match (use BM25), 매 small corpus (<1k docs — 매 LLM-direct 가 simpler), 매 high-precision regex needs.

## ❌ 안티패턴
- **Dense-only**: 매 BM25 매 still wins on rare terms / proper nouns — 매 hybrid.
- **No reranker**: 매 top-10 quality 매 leaves 30% on table.
- **Bad chunking**: 매 fixed-size mid-sentence — 매 use semantic / heading-aware.
- **No metadata filter**: 매 hybrid filter (date/source) before vector search.
- **Cosine without normalize**: 매 silent bug — 매 always normalize L2.

## 🧪 검증 / 중복
- Verified (Karpukhin DPR 2020, Khattab ColBERT 2020, MTEB benchmark, Cohere Rerank docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — hybrid, MRL, ColBERT, pgvector, multimodal |

## 🛠️ 적용 사례 (Applied in summary)

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### 🔎 코드베이스 근거 (자동 추출 — E:\Wiki 레포)
**실제 구현/사용 위치:**
- `connectai/src/features/projectChronicle/guardPrompt.ts:57` — [Omitted long matching line]

_자동 생성: code_grounding.mjs · 재실행 시 갱신됨_
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