XML query optimization in the presence of side effects

The emergence of database languages with side effects, notably for XML, raises significant challenges for database compilers and optimizers. In this paper, we extend an algebra for the W3C XML query language with operations that allow data to be immediately updated. We study the impact of that extension on logical optimization, join detection, and pipelining. The main result of this work is to show that, with proper care, a number of important optimizations based on nested relational algebras remain applicable in the presence of side effects. Our approach relies on an analysis of the conditions that must be checked in order for algebraic rewritings to hold. An implementation and experimental results demonstrate the effectiveness of the approach.

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