What are the differences between exact and approximate vector search?

Exact and approximate vector search are two approaches for finding similar vectors in a dataset, differing primarily in accuracy, speed, and scalability. Exact vector search guarantees precise results by comparing a query vector to every vector in the dataset, ensuring the closest matches are found. This method is exhaustive and deterministic, meaning it always returns the same results for the same query. However, it becomes computationally expensive as dataset size grows, making it impractical for large-scale applications. Approximate vector search, in contrast, sacrifices some accuracy for speed by using techniques that reduce the number of comparisons needed. These methods return “good enough” results quickly, even for massive datasets, but may miss some of the closest matches.

The technical differences lie in their algorithms. Exact methods, like linear search or k-d trees, perform brute-force comparisons or structured searches that partition data to avoid checking every vector. For example, a k-d tree organizes vectors in a binary tree structure, allowing faster exact searches in lower-dimensional spaces. Approximate methods, such as locality-sensitive hashing (LSH) or hierarchical navigable small world (HNSW) graphs, use probabilistic or graph-based strategies to narrow down candidates. LSH, for instance, hashes similar vectors into the same “buckets,” reducing the search space. HNSW builds a layered graph where traversal efficiently approximates nearest neighbors. Libraries like FAISS or Annoy implement these approximate techniques, optimizing for speed and memory usage while accepting small errors in results.

Use cases dictate which approach to choose. Exact search is ideal when precision is critical, such as medical image analysis or fraud detection, where missing a match could have serious consequences. However, it’s limited to smaller datasets (e.g., thousands of vectors) due to its O(n) time complexity. Approximate search suits applications like recommendation systems or real-time semantic search in large databases (millions of vectors), where latency matters more than perfect accuracy. For example, an e-commerce platform might use approximate search to quickly find product recommendations, accepting that some relevant items might be overlooked. Developers should balance trade-offs: exact methods for accuracy at the cost of scalability, approximate methods for speed with a tolerable error margin. Hybrid approaches, like exact search on filtered subsets, can also bridge these extremes.