Query completion systems are well studied in the context of information retrieval systems that handle keyword queries. However, Natural Language Interface to Databases (NLIDB) systems that focus on syntactically correct and semantically complete queries to obtain high precision answers require a fundamentally different approach to the query completion problem as opposed to IR systems. To the best of our knowledge, we are first to focus on the problem of query completion for NLIDB systems. In particular, we introduce a novel concept of functional partitioning of an ontology and then design algorithms to intelligently use the components obtained from functional partitioning to extend a state-of-the-art NLIDB system to produce accurate and semantically meaningful query completions in the absence of query logs. We test the proposed query completion framework on multiple benchmark datasets and demonstrate the efficacy of our technique empirically.