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XML query can be modeled by twig pattern query (TPQ) specifying predicates onXML nodes and XPath relationships satisfied between them. A lot of TPQ typeshave been proposed; this paper takes into account a TPQ model extended by aspecification of output and non-output query nodes since it complies with theXQuery semantics and, in many cases, it leads to a more efficient queryprocessing. In general, there are two approaches to process the TPQ: holisticjoins and binary joins. Whereas the binary join approach builds a query plan asa tree of interconnected binary operators, the holistic join approach evaluatesa whole query using one operator (i.e., using one complex algorithm).Surprisingly, a thorough analytical and experimental comparison is stillmissing despite an enormous research effort in this area. In this paper, we tryto fill this gap; we analytically and experimentally show that the binary joinsused in a fully-pipelined plan (i.e., the plan where each join operation doesnot wait for the complete result of the previous operation and no explicitsorting is used) can often outperform the holistic joins, especially for TPQswith a higher ratio of non-output query nodes. The main contributions of thispaper can be summarized as follows: (i) we introduce several improvements ofexisting binary join approaches allowing to build a fully-pipelined plan for aTPQ considering non-output query nodes, (ii) we prove that for a certain classof TPQs such a plan has the linear time complexity with respect to the size ofthe input and output as well as the linear space complexity with respect to theXML document depth (i.e., the same complexity as the holistic join approaches),(iii) we show that our improved binary join approach outperforms the holisticjoin approaches in many situations, and (iv) we propose a simple combinedapproach that uses advantages of both types of approaches.

Demythization of Structural XML Query Processing: Comparison of Holistic and Binary Approaches, Technical Report