SchemaScope: How Join-Hop Depth Breaks Text-to-SQL in Large Language Models, and a Decomposition-Based Remedy
Large language models (LLMs) achieve impressive accuracy on standard Text-to-SQL benchmarks such as Spider and BIRD, yet enterprise databases—with hundreds of tables and complex foreign key graphs—remain a practical bottleneck. We hypothesise that a single, measurable property drives most of this gap: the join-hop depth ( h ) of the query, defined as the number of foreign key edges that must be traversed to gather all required columns. We introduce the Join-Hop Depth (JHD) benchmark , 410 human-annotated questions stratified by h ∈ { 1 , . . . , 6 } over 12 enterprise-scale schemas. Experiments on five frontier LLMs confirm a sharp accuracy cliff: all models exceed 80% at h =1 but fall below 40% at h =4 and below 25% at h =6 —the typical depth of real enterprise analytics queries. To address this, we propose S CHEMA S COPE , a decomposition framework that partitions deep queries into a sequence of sub-queries with h ≤ 2 , executes them independently, and merges the re-sults. S CHEMA S COPE raises average execution accuracy from 46.8% to 67.3% on JHD (GPT-4o, h ≥ 3 ) and improves execution accuracy by +9 . 3 pp on the BIRD development set. Error analysis shows that decomposition eliminates wrong join path errors—the dominant failure mode at high h —and shifts the residual error budget toward condition and aggregation mistakes that are amenable to existing post-processing methods