Learning Notes

FAISS

f## 1. What FAISS is FAISS (Facebook AI Similarity Search) is a library for fast similarity search over vector embeddings. It retrieves nearest vectors to a query vector using different indexing strategies.

2. Core Idea

Workflow: Text → Embedding → Vector Index → Similarity Search → Top-K Results → Map to original text (RAG retrieval)

3. Indexes (Main Search Engine Types)

A. Flat Indexes (Exact Search)

  • IndexFlatIP → Inner Product (cosine similarity when normalized)
  • IndexFlatL2 → Euclidean distance
  • Behavior: brute-force scan of all vectors
  • Pros: exact results
  • Cons: slow at scale (O(N))

B. IVF Indexes (Cluster-Based Approximate Search)

  • IndexIVFFlat → clusters + exact search inside selected clusters
  • IndexIVFPQ → IVF + compression (Product Quantization)
  • IndexIVFSQ → scalar quantization variant

Mechanism:

  1. Cluster vectors (k-means)
  2. Assign vectors to centroids
  3. Query searches only nearest clusters (controlled by nprobe)

Tradeoff: speed vs accuracy

C. HNSW Index (Graph-Based Search)

  • IndexHNSWFlat Mechanism:
  • Builds graph of vector neighbors
  • Search by traversing closest nodes

Pros:

  • Very fast
  • High recall Cons:
  • Memory heavy

D. Compression Indexes

  • PQ (Product Quantization)
  • IVFPQ (IVF + PQ compression)

Purpose:

  • Reduce memory footprint
  • Store compressed vector codes instead of full floats

4. ID Wrappers (Output Mapping Layer)

IndexIDMap

Maps FAISS internal positions → real-world IDs (chunk_id, doc_id) Without:

  • returns 0,1,2,...

With:

  • returns meaningful IDs for RAG retrieval

IndexIDMap2

Advanced version with better flexibility for dynamic updates

Key point:

  • Does NOT affect search, only output identity

5. Training Component

Required for:

  • IVF (clustering)
  • PQ (codebook learning)

Function:

  • index.train(data)

What it does:

  • Learns clusters (IVF)
  • Learns quantization codebooks (PQ)

6. Vector Transformations (Preprocessing)

  • Normalization → unit length vectors (enables cosine via inner product)
  • PCA → dimensionality reduction
  • OPQ → optimized rotation for better compression

7. Similarity Metrics

  • Inner Product (IP): used for cosine similarity when vectors are normalized
  • L2 Distance: Euclidean distance

8. Search Optimization Parameters

  • IVF: nprobe → number of clusters searched
  • HNSW: efSearch → graph traversal depth

Higher = more accurate, slower

9. GPU Acceleration

  • GPU IndexFlatIP / GPU IVF
  • index_cpu_to_gpu()

Used for:

  • large-scale / high-throughput inference

10. Index Factory

faiss.index_factory()

Examples:

  • "Flat"
  • "IVF100,Flat"
  • "IVF100,PQ16"
  • "HNSW32"

Purpose:

  • declarative index creation

11. Persistence Layer

  • write_index()
  • read_index()

Used to save/load FAISS indexes in production

12. Metadata Layer (External System)

FAISS does NOT store text.

You must maintain:

  • chunk text
  • document IDs
  • metadata

IndexIDMap connects vectors → metadata system

13. Full RAG Flow

  1. Chunk text
  2. Embed (BGE-m3 or similar)
  3. Normalize vectors
  4. Build FAISS index
    • Flat / IVF / HNSW
    • optionally IDMap
  5. Query embedding
  6. FAISS search
  7. Retrieve chunk IDs
  8. Fetch actual text from DB

14. Big Picture Mental Model

  • Flat → brute force exact search
  • IVF → clustered approximate search
  • HNSW → graph traversal search
  • PQ → compressed storage
  • IDMap → identity mapping layer
  • Training → structure learning step
  • GPU → acceleration layer

15. Key Engineering Tradeoffs

  • Flat: accuracy ↑, speed ↓
  • IVF: balanced
  • HNSW: fast + accurate, memory heavy
  • PQ: memory efficient, less accurate