Parallel-Correctness and Transferability for Conjunctive Queries under Bag Semantics

Single-round multiway join algorithms first reshuffle data over many servers and then evaluate the query at hand in a parallel and communication-free way. A key question is whether a given distribution policy for the reshuffle is adequate for computing a given query. This property is referred to as parallel-correctness. Another key problem is to detect whether the data reshuffle step can be avoided when evaluating subsequent queries. The latter problem is referred to as transfer of parallel-correctness. This paper extends the study of parallel-correctness and transfer of parallel-correctness of conjunctive queries to incorporate bag semantics. We provide semantical characterizations for both problems, obtain complexity bounds and discuss the relationship with their set semantics counterparts. Finally, we revisit both problems under a modified distribution model that takes advantage of a linear order on compute nodes and obtain tight complexity bounds

DPOS: A metalanguage and programming environment for parallel processors

Journal ArticleThe complexity and diversity of parallel programming languages and computer architectures hinders programmers in developing programs and greatly limits program portability. All MIMD parallel programming systems, however, address common requirements for process creation, process management, and interprocess communication. This paper describes and illustrates a structured programming system (DPOS) and graphical programming environment for generating and debugging high-level MIND parallel programs. DPOS is a metalanguage for defining parallel program networks based on the common requirements of distributed parallel computing that is portable across languages, modular, and highly flexible. The system uses the concept of stratification to separate process network creation and the control of parallelism form computational work. Individual processes are defined within the process object layer as traditional single threaded programs without parallel language constructs. Process networks and communication are defined graphically within the system layer at a high level of abstraction as recursive graphs. Communication is facilitated in DPOS by extending message passing semantics in several ways to implement highly flexible message passing constructs. DPOS processes exchange messages through bi-directional channel objects using guarded, buffered, synchronous and asynchronous communication semantics. The DPOS environment also generates source code and provides a simulation system for graphical debugging and animation of the programs in graph form