HEXA: A Disjoint-Subgraph-Based Indexing Framework for Approximate Nearest Neighbor Search at Billion Scale

Approximate Nearest Neighbor (ANN) search is a core operation in large-scale vector databases that support recommendation, semantic search, and retrieval-augmented generation. Graph-based indexes are widely used for their superior search performance at million scale, but scaling them to billion-scale datasets exposes a fundamental trade-off between construction efficiency and search performance. CPU-based methods produce high-quality graphs enabling fast search but require multiple days to construct, while GPU-based methods accelerate construction at the cost of degraded search performance. This trade-off stems from the traditional requirement of enforcing global connectivity during graph construction. We observe that graph-based search primarily depends on local connectivity between entry points and a query's nearest neighbors, making global connectivity unnecessary for high search performance. In this paper, we introduce HEXA, a disjoint-subgraph-based indexing framework that partitions the dataset into disjoint clusters and constructs refined proximity subgraphs independently on GPUs. At query time, HEXA employs a two-level routing structure to select relevant subgraphs and query-close entry points, then applies a budget-adaptive search algorithm that allocates exploration effort based on each subgraph's likelihood of containing nearest neighbors. Experiments on billion-scale datasets demonstrate that HEXA achieves up to 14.7 times higher query throughput at the same recall levels compared with various indexing methods, and reduces construction time to approximately one hour.