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Monday, 12 February 2018

Extended XML Tree Pattern Matching Theories and Algorithms(2011)

Extended XML Tree Pattern Matching 

Theories and Algorithms(2011)

As business and enterprises generate and exchange XML data more often, there is an increasing need for efficient processing of queries on XML data. Searching for the occurrences of a tree pattern query in an XML database is a core operation in XML query processing. Prior works demonstrate that holistic twig pattern matching algorithm is an efficient technique to answer an XML tree pattern with parent-child (P-C) and ancestor-descendant (A-D) relationships, as it can effectively control the size of intermediate results during query processing. However, XML query languages (e.g. XPath, XQuery) define more axes and functions such as negation function, order-based axis and wildcards. Here we research a large set of XML tree pattern, called extended XML tree pattern, which may include P-C, A-D relationships, negation functions, wildcards and order restriction. We establish a theoretical framework about “matching cross” which demonstrates the intrinsic reason in the proof of optimality on holistic algorithms. Based on our theorems, we propose a set of novel algorithms to efficiently process three categories of extended XML tree patterns. A set of experimental results on both real-life and synthetic data sets demonstrate the effectiveness and efficiency of our proposed theories and algorithms.
Existing System:
Previous algorithms focus on XML tree pattern queries with only P-C and A-D relationships. Little work has been done on XML tree queries which may contain wildcards, negation function and order restriction, all of which are frequently used in XML query languages such as XPath and Xquery. In this article, we call an XML tree pattern with negation function, wildcards and/or order restriction as extended XML tree pattern. Previous XML tree pattern matching algorithms do not fully exploit the “optimality” of holistic algorithms.
Proposed System:
We build a theoretical framework on optimal processing of XML tree pattern queries. We show that “matching cross” is the key reason to result in the sub-optimality of holistic algorithms. Intuitively, matching cross describes a dilemma where holistic algorithms have to decide whether to output useless intermediate result or to miss useful results. The fact that TwigStack is optimal for queries with only A-D relationships can be explained that no matching cross can be found for any XML document with respect to queries with A-D edges. We classify matching cross to bound and unbounded matching cross (BMC and UMC). We develop theorems to show that only part of UMC (i.e. UMC with mediator) can force holistic algorithms to potentially output useless intermediate results.  Based on the theoretical analysis, we develop a series of holistic algorithms TreeMatch to achieve a large optimal query class for Q/,//,*. Our main technique is to use a concise encoding to present matching results, which leads to the reduction of useless intermediate results. We conducted an extensive set of experiment on synthetic and real data set for performance comparison. We compared TreeMatch with previous four holistic XML tree pattern matching algorithms. The experimental results show that our algorithm can correctly process extended XML tree patterns, achieving performance speedup for tested queries and data sets, even in their restricted focus. The improvement mainly owes to the reduction of the size of intermediate results.
1. Optimality of holistic algorithm:
Previous XML tree pattern matching algorithms do not fully exploit the “optimality” of holistic algorithms. TwigStack guarantees that there is no useless intermediate result for queries with only Ancestor-Descendant (A-D) relationships. Therefore, TwigStack is optimal for queries with only A-D edges. Another algorithm TwigStackList  enlarges the optimal query class of TwigStack by including Parent-Child(P-C) relationships in non-branching edges. A natural question is whether the optimal query class of TwigStackList can be further improved. Hence, the current open problems include (1) how to identify a larger query class which can be processed optimally and (2) how to efficiently answer a query which cannot be guaranteed to process optimally.  This explores the challenges and shows the promise of a novel  theoretical framework called “matching cross” to identify a large optimal query class for posing extended XML tree queries.
2. Return nodes in twig pattern queries:
In a practical application, only part of query nodes belong to return nodes (or called output nodes interchangeably). Take the XPath “//A[B]//C” as an example, only element and its subtree are answers. The current “modus operandi” is that they first find the query answer with the combinations of all query nodes, and then do an appropriate projection on those return nodes. Such a post-processing approach has an obvious disadvantage: it outputs many matching elements of non-return nodes that are unnecessary for the final results. Here, we develop a new encoding method to record the mapping relationships and avoid outputting non-return nodes.
3. Modeling of XML data and extended tree pattern query:
An XML database is usually modeled as a rooted, node labeled tree, element tags and attributes are mapped to nodes in the trees and the edges are used to represent the direct nesting relationships. Our primary focus is on element nodes; and it is not difficult to extend our methods to process the other types of nodes, including attribute and character data. For convenience, we distinguish between query nodes and database nodes by using the term “node” to refer to a query node and the term “element” to refer to a data element in D. An extended tree query describes a complex traversal of the XML document and retrieves relevant tree-structured portions of it. The nodes in include element tags, attributes and character data. We use “*” to denote the wildcard, which can match any single tree element. There are four kinds of query edges, which are the four combinations between (positivenegative) and (parent-childancestor-descendant).
4. Matching Cross:
“Matching cross” demonstrates the intrinsic reason for the sub-optimality of existing holistic algorithms. The purposes of our study are (i) to provide insight into the characteristics of the holistic algorithms, and thus promotes our understanding about their behaviors; and (ii) to lead to novel algorithms that can guarantee a larger optimal query class and realize better query performance. The existing holistic algorithms consist of two phases: (i) in the first phase, a list of path solutions is output as intermediate path solutions and each solution matches the individual root-to-leaf path expression; and (ii) in the second phase, the path solutions are merged to produce the final answers for the whole twig query. However, for queries with parent-child (P-C) relationships, the state-of-the-art algorithms cannot guarantee that each intermediate solution output in the first phase is useful to merge in the second phase. In other words, many useless intermediate results (i.e. path solutions) may be produced in the first phase, which is called the suboptimality of algorithms.
H/W System Configuration:-
ü Processor             -Pentium –III
ü Speed                             -    1.1 Ghz
ü RAM                    -    256 MB(min)
ü Hard Disk            -   20 GB
ü Floppy Drive       -    1.44 MB
ü Key Board            -    Standard Windows Keyboard
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ü Monitor                -    SVGA
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v   Operating System          : Windows95/98/2000/XP
v   Application  Server       : Tomcat5.0/6.X  
v   Front End                      : HTML, Java, JSP
v    Script                            : JavaScript.
v   Server side Script          : Java Server Pages.
v   Database                        : Ms-Access
v   Database Connectivity   : JDBC.

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