  Big data management aims to establish data hubs that support data in multiple
models and types in an all-around way. Thus, the multi-model database system is
a promising architecture for building such a multi-model data store. For an
integrated data hub, a unified and flexible query language is incredibly
necessary. In this paper, an extensible and practical query language--Multi-SQL
is proposed to realize the unified management of multi-model data considering
the co-processing of multi-model data. To the best of our knowledge, Multi-SQL
is the first query language based on various data models. Multi-SQL can also be
expanded to suit more complicated scenarios as it is flexible to support other
data models. Moreover, we provide a formal semantic definition of the core
features of Multi-SQL, including the multi-model definition, multi-model
filters, multi-model joins, etc. Furthermore, we propose a two-level query
implementation method to totally exploit the existing query optimization
capabilities of the underlying engines which could largely improve the query
excution efficiency.
