XQuery is a feature-rich language with complex semantics. This makes it hard to come up with a benchmark suite which covers all performance-critical features of the language, and at the same time allows one to individually validate XQuery evaluation techniques. This paper presents MemBeR, a micro-benchmark repository, allowing the evaluation of an XQuery implementation with respect to precise evaluation techniques. We take the view that a fixed set of queries is probably insufficient to allow testing for various performance aspects, thus, the users of the repository must be able to add new data sets and/or queries for specific performance assessment tasks. We present our methodology for constructing the micro-benchmark repository, and illustrate with some sample micro-benchmarks.
We present a sound and complete rule set for determining whether sorting and duplicate removal operations in the query plan of XPath expressions are unnecessary. Additionally we define a deterministic finite automaton that illustrates how these rules can be translated into an efficient algorithm. This work is an important first step in the understanding and tackling of XPath/XQuery optimization problems that are related to ordering and duplicate removal.
XQuery expressions can manipulate two kinds of order: document order and sequence order. While the user can impose or observe the order of items within a sequence, the results of path expressions must always be returned in document order. Correctness can be obtained by inserting explicit (and expensive) operations to sort and remove duplicates after each XPath step. However, many such operations are redundant. In this paper, we present a systematic approach to remove unnecessary sorting and duplicate elimination operations in path expressions in XQuery 1.0. The technique uses an automaton-based algorithm which we have applied successfully to path expressions within a complete XQuery implementation. Experimental results show that the algorithm detects and eliminates most redundant sorting and duplicate elimination operators and is very effective on common XQuery path expressions.
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