Tractable Iteration Mechanisms for Bag Languages

Abstract. 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 de ationary xpoint construct keeps removing elements from a bag until a xpoint is reached. The bounded xpoint construct is an in ationary iteration mechanism that never exceeds some prede ned bounding bag. We study these constructs in the context of a standard (nested) bag algebra. We show that the deationary and bounded in ationary 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...

• Type: Conference
• Year: 1997
• Where: ICDT
• Authors: Latha S. Colby, Leonid Libkin
• Keywords: Ationary Xpoint Constructs, Database, ICDT 1997, Iteration Mechanism, Xpoint Constructs