1. A method for prioritizing queries in a cloud based database, comprising: determining a priority score in real time under an imprecise query execution time; and incrementally maintaining query priority scores; and managing the cloud based database using a processor to schedule queries with different service level agreements and imprecise query execution time using robust cost-based scheduling (rCBS), that does not require prior knowledge about the exact query execution time, wherein the rCBS explicitly evaluates all values of an execution time of a query and corresponding costs to handle queries with step-wise service level agreements (SLAs) functions in an O(log N) time complexity, where N is the number of queries are to be scheduled; and mapping a time-varying score for a query to a fixed point in a dual space of line function as 
 p q ( t )=α q +β q e bt , where α q and β q constants invariant with respect to t for a certain period of time, wherein t is time and q is query; and providing complexity/faster operation highly scalable rCBS for leveraging dynamic convex hull algorithms and the principle of superposition, which result in a logarithmic time complexity, and therefore it can efficiently handle large number of queries with different the SLAB in real time.