What are the common challenges in vector search?

Vector search faces three primary challenges: handling scalability with large datasets, maintaining accuracy in search results, and balancing efficiency in storage and computation. Each of these areas requires careful design choices and trade-offs to build effective systems.

The first challenge is scalability. As datasets grow to millions or billions of vectors, traditional search methods become impractical. For example, exact nearest neighbor algorithms like brute-force search have a time complexity that scales linearly with the dataset size, making them too slow for real-time applications. Approximate Nearest Neighbor (ANN) algorithms, such as HNSW (Hierarchical Navigable Small World) or IVF (Inverted File Index), improve speed by sacrificing some accuracy. However, distributing these algorithms across multiple servers introduces complexity. Sharding data across nodes can help, but it requires managing consistency, load balancing, and failover mechanisms. For instance, a recommendation system with 100 million user embeddings might use ANN with distributed sharding, but network latency and synchronization between nodes could still impact performance.

The second challenge is ensuring accuracy. Vector search relies on embeddings—numeric representations of data—to measure similarity. If the embedding model isn’t trained properly, search quality suffers. For example, a poorly trained image embedding model might group unrelated images (e.g., cats and cars) as “similar” due to color patterns. Additionally, high-dimensional vectors (e.g., 768 dimensions in BERT embeddings) can lead to the “curse of dimensionality,” where distances between vectors become less meaningful. Techniques like dimensionality reduction (e.g., PCA) or using domain-specific distance metrics (e.g., cosine similarity for text) help, but they require experimentation. For example, in e-commerce, product search might need custom metrics that prioritize price or brand over visual similarity.

The third challenge is balancing storage and computational efficiency. Storing billions of vectors demands significant memory, especially when using high-precision formats like 32-bit floats. Compression methods like quantization (e.g., converting 32-bit floats to 8-bit integers) reduce memory usage but introduce approximation errors. Real-time search latency is another concern: even with ANN, querying large indexes can take milliseconds, which may not meet strict service-level agreements. Updating indexes incrementally without full rebuilds adds complexity. For example, a social media platform adding new user posts daily might use delta indexing, but over time, fragmented indexes could degrade search speed. Optimizing these trade-offs often requires hardware-specific tuning, such as leveraging GPUs for faster distance calculations or using in-memory databases for low-latency access.