Iteration Mechanisms
نویسندگان
چکیده
The goal of this paper is to study tractable iteration mechanisms for bags. The presence of duplicates in bags prevents iteration mechanisms developed in the context of sets to be directly applied to bags without losing tractability. We study two constructs for controlling tractability of iteration over bags. The deeationary xpoint construct keeps removing elements from a bag until a xpoint is reached. The bounded xpoint construct is an innationary iteration mechanism that never exceeds some predeened bounding bag. We study these constructs in the context of a standard (nested) bag algebra. We show that the de-ationary and bounded innationary xpoint constructs are equally expressive and strictly more expressive than their set-based counterparts. We also show that, unlike in the set case, the bag algebra with bounded xpoint fails to capture all PTIME queries over databases with ordered domains. We then show that adding just one construct, which can be used to assign unique tags to duplicates, captures the class of all polynomial time queries over bags when a total ordering on the domain of atomic elements is available. Finally, we compare the expressive powers of the bag algebra and the nested relational algebra with aggregate functions in the presence of these xpoint operators.
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تاریخ انتشار 2009