Query Load Balancing in Parallel Database Systems

International audienceDefinition : The goal of parallel query execution is minimizing query response time using inter- and intraoperator parallelism. Interoperator parallelism assigns different operators of a query execution plan to distinct (sets of) processors, while intraoperator parallelism uses several processors for the execution of a single operator, thanks to data partitioning. Conceptually, parallelizing a query amounts to divide the query work in small pieces or tasks assigned to different processors. The response time of a set of parallel tasks being that of the longest one, the main difficulty is to produce and execute these tasks such that the query load is evenly balanced within the processors. This is made more complex by the existence of dependencies between tasks (e.g., pipeline parallelism) and synchronizations points. Query load balancing relates to static and/or dynamic techniques and algorithms to balance the query load within the processors so that the response time is minimized