FAISS Index Types Guide

Complete guide to choosing and using FAISS index types.

Index selection guide

Flat indices (exact search)

IndexFlatL2 - L2 (Euclidean) distance

import faiss
import numpy as np

d = 128  # Dimension
index = faiss.IndexFlatL2(d)

# Add vectors
vectors = np.random.random((1000, d)).astype('float32')
index.add(vectors)

# Search
k = 5
query = np.random.random((1, d)).astype('float32')
distances, indices = index.search(query, k)

Use when:

• Dataset < 10,000 vectors
• Need 100% accuracy
• Serving as baseline

IndexFlatIP - Inner product (cosine similarity)

# For cosine similarity, normalize vectors first
import faiss

d = 128
index = faiss.IndexFlatIP(d)

# Normalize vectors (required for cosine similarity)
faiss.normalize_L2(vectors)
index.add(vectors)

# Search
faiss.normalize_L2(query)
distances, indices = index.search(query, k)

Use when:

• Need cosine similarity
• Recommendation systems
• Text embeddings

IVF indices (inverted file)

IndexIVFFlat - Cluster-based search

# Create quantizer
quantizer = faiss.IndexFlatL2(d)

# Create IVF index with 100 clusters
nlist = 100  # Number of clusters
index = faiss.IndexIVFFlat(quantizer, d, nlist)

# Train on data (required!)
index.train(vectors)

# Add vectors
index.add(vectors)

# Search (nprobe = clusters to search)
index.nprobe = 10  # Search 10 closest clusters
distances, indices = index.search(query, k)

Parameters:

• nlist: Number of clusters (√N to 4√N recommended)
• nprobe: Clusters to search (1-nlist, higher = more accurate)

Use when:

• Dataset 10K-1M vectors
• Need fast approximate search
• Can afford training time

Tuning nprobe

# Test different nprobe values
for nprobe in [1, 5, 10, 20, 50]:
    index.nprobe = nprobe
    distances, indices = index.search(query, k)
    # Measure recall/speed trade-off

Guidelines:

• nprobe=1: Fastest, ~50% recall
• nprobe=10: Good balance, ~95% recall
• nprobe=nlist: Exact search (same as Flat)

HNSW indices (graph-based)

IndexHNSWFlat - Hierarchical NSW

# HNSW index
M = 32  # Number of connections per layer (16-64)
index = faiss.IndexHNSWFlat(d, M)

# Optional: Set ef_construction (build time parameter)
index.hnsw.efConstruction = 40  # Higher = better quality, slower build

# Add vectors (no training needed!)
index.add(vectors)

# Search
index.hnsw.efSearch = 16  # Search time parameter
distances, indices = index.search(query, k)

Parameters:

• M: Connections per layer (16-64, default 32)
• efConstruction: Build quality (40-200, higher = better)
• efSearch: Search quality (16-512, higher = more accurate)

Use when:

• Need best quality approximate search
• Can afford higher memory (more connections)
• Dataset 1M-10M vectors

PQ indices (product quantization)

IndexPQ - Memory-efficient

# PQ reduces memory by 16-32×
m = 8   # Number of subquantizers (divides d)
nbits = 8  # Bits per subquantizer

index = faiss.IndexPQ(d, m, nbits)

# Train (required!)
index.train(vectors)

# Add vectors
index.add(vectors)

# Search
distances, indices = index.search(query, k)

Parameters:

• m: Subquantizers (d must be divisible by m)
• nbits: Bits per code (8 or 16)

Memory savings:

• Original: d × 4 bytes (float32)
• PQ: m bytes
• Compression ratio: 4d/m

Use when:

• Limited memory
• Large datasets (> 10M vectors)
• Can accept ~90-95% accuracy

IndexIVFPQ - IVF + PQ combined

# Best for very large datasets
nlist = 4096
m = 8
nbits = 8

quantizer = faiss.IndexFlatL2(d)
index = faiss.IndexIVFPQ(quantizer, d, nlist, m, nbits)

# Train
index.train(vectors)
index.add(vectors)

# Search
index.nprobe = 32
distances, indices = index.search(query, k)

Use when:

• Dataset > 10M vectors
• Need fast search + low memory
• Can accept 90-95% accuracy

GPU indices

Single GPU

import faiss

# Create CPU index
index_cpu = faiss.IndexFlatL2(d)

# Move to GPU
res = faiss.StandardGpuResources()  # GPU resources
index_gpu = faiss.index_cpu_to_gpu(res, 0, index_cpu)  # GPU 0

# Use normally
index_gpu.add(vectors)
distances, indices = index_gpu.search(query, k)

Multi-GPU

# Use all available GPUs
index_gpu = faiss.index_cpu_to_all_gpus(index_cpu)

# Or specific GPUs
gpus = [0, 1, 2, 3]  # Use GPUs 0-3
index_gpu = faiss.index_cpu_to_gpus_list(index_cpu, gpus)

Speedup:

• Single GPU: 10-50× faster than CPU
• Multi-GPU: Near-linear scaling

Index factory

# Easy index creation with string descriptors
index = faiss.index_factory(d, "IVF100,Flat")
index = faiss.index_factory(d, "HNSW32")
index = faiss.index_factory(d, "IVF4096,PQ8")

# Train and use
index.train(vectors)
index.add(vectors)

Common descriptors:

• "Flat": Exact search
• "IVF100,Flat": IVF with 100 clusters
• "HNSW32": HNSW with M=32
• "IVF4096,PQ8": IVF + PQ compression

Performance comparison

Search speed (1M vectors, k=10)

CPU (16 cores), 128-dim vectors

Best practices

1. Start with Flat - Baseline for comparison
2. Use IVF for medium datasets - Good balance
3. Use HNSW for best quality - If memory allows
4. Add PQ for memory savings - Large datasets
5. GPU for > 100K vectors - 10-50× speedup
6. Tune nprobe/efSearch - Trade-off speed/accuracy
7. Train on representative data - Better clustering
8. Save trained indices - Avoid retraining

Resources

• Wiki: https://github.com/facebookresearch/faiss/wiki
• Paper: https://arxiv.org/abs/1702.08734