Demythization Of Structural Xml Query Processing: Comparison Of Holistic And Binary Approaches

Structural XQuery and XPath queries are often modeled by twig pattern queries (TPQs) specifying predicates on XML nodes and structural relationships to be satisfied between them. This paper considers a TPQ model extended by a specification of output and non-output query nodes since it complies with the XQuery and XPath semantics. There are two types of TPQ processing approaches: binary joins and holistic joins. The binary joins utilize a query plan of interconnected binary operators, whereas the holistic joins are based on one complex operator to process the whole query. In the recent years, the holistic joins have been considered as the state-of-the-art TPQ processing method. However, a thorough analytical and experimental comparison of binary and holistic joins has been missing despite an enormous research effort in this area. In this paper, we try to fill this gap. We introduce several improvements of the binary join operators which enable us to build a so-called fully-pipelined (FP) query plan for any TPQ with the specification of output and non-output query nodes. We analytically show that, for a class of queries, the proposed approach has the same time and space complexity as holistic joins, and we experimentally demonstrate that the proposed approach outperforms holistic joins in many cases.