1. A computer system comprising: a first computer having a first memory and being capable of executing a query; and a second computer having a second memory and being capable of executing said query, wherein said first computer: when stream data is received from an external device via a network, said query is to be executed and said stream data is to be processed based on said query, judges whether information on a sliding window used for processing said stream data exists on said first memory; if said information on said sliding window exists on said first memory, executes processing of said stream data based on said query using said information on said sliding window; and if said information on said sliding window does not exist on said first memory, transmits an identifier contained in said received stream data to said second computer; receives from said second computer said information on sliding window corresponding to said identifier; and executes processing of said stream data based on said query using said information on sliding window received from said second computer, wherein said second computer transmits said information on said sliding window corresponding to said identifier received from said first computer.