The term "Big Data" became a buzzword and is widely used in both research and industrial worlds. Typically the concept of big data assumes a variety of different sources of information and velocity of complex analytical processing, rather than just a huge and growing volume of data. All variety, velocity, and volume create new research challenges, as nearly all techniques and tools commonly used in data processing have to be re-considered. Variety and uncertainty of big data require a mixture of exact and similarity search and grouping of complex objects based on different attributes. High-level declarative query languages are important in this context due to expressiveness and potential for optimization. In this talk we are mostly interested in an algebraic layer for complex query processing which resides between user interface (most likely, graphical) and execution engine in layered system architecture. We analyze the applicability of existing models and query languages. We describe a systematic approach t