Outlining and Filling: Hierarchical Query Graph Generation for Answering Complex Questions over Knowledge Graph

Query graph building aims to build correct executable SPARQL over the knowledge graph for answering natural language questions. Although recent approaches perform well by NN-based query graph ranking, more complex questions bring three new challenges: complicated SPARQL syntax, huge search space for ranking, and noisy query graphs with local ambiguity. This paper handles these challenges. Initially, we regard common complicated SPARQL syntax as the sub-graphs comprising of vertices and edges and propose a new unified query graph grammar to adapt them. Subsequently, we propose a new two-stage approach to build query graphs. In the first stage, the top-$k$ related instances (entities, relations, etc.) are collected by simple strategies, as the candidate instances. In the second stage, a graph generation model performs hierarchical generation. It first outlines a graph structure whose vertices and edges are empty slots, and then fills the appropriate instances into the slots, thereby completing the query graph. Our approach decomposes the unbearable search space of entire query graphs into affordable sub-spaces of operations, meanwhile, leverages the global structural information to eliminate local ambiguity. The experimental results demonstrate that our approach greatly improves state-of-the-art on the hardest KGQA benchmarks and has an excellent performance on complex questions.