An encoding scheme based on fractional number for querying and updating XML data

Abstract

In order to facilitate the XML query processing, several labeling schemes have been proposed to directly determine the structural relationships between two arbitrary XML nodes without accessing the original XML documents. However, the existing XML labeling schemes have to re-label the pre-existing nodes or re-calculate the label values when a new node is inserted into the XML document during an update process. In this paper, we devise a novel encoding scheme based on the fractional number to encode the labels of the XML nodes. Moreover, we propose a mapping method to convert our proposed fractional number based encoding scheme to bit string based encoding scheme with the intention to minimize the label size and save the storage space. By applying our proposed bit string encoding scheme to the range-based labeling scheme and the prefix labeling scheme, the process of re-labeling the pre-existing nodes can be avoided when nodes are inserted as leaf nodes and sibling nodes without affecting the order of XML nodes. In addition, we propose an algorithm to control the increment of label size when new nodes are inserted frequently at a fix place of an XML tree. Experimental results show that our proposed bit string encoding scheme provides efficient support to the process of XML updating without sacrificing the query performance when it is applied to the range-based labeling schemes.

Introduction

With the growing popularity of XML for representing and exchanging data over the Internet, the need for systems which can store and query the XML data efficiently seems necessary. Several query languages like XPath (Clark and DeRose, 1999) and XQuery (Boag et al., 2007) have been proposed to process the XML data. These query languages are based on the regular path expression to query the XML data. In order to facilitate the XML query processing, the structural relationships between XML nodes should be directly determined without accessing the XML documents. Several researches have been done to label the XML nodes in such a way that the structural relationships between two arbitrary nodes can be determined which include studies by Amagasa et al. (2003), Cohen et al. (2002), Li and Ling (2005), Li et al., 2006, Li et al., 2008, Li and Moon (2001), Min et al., 2007, Min et al., 2009, O’Neil et al. (2004), Tatarinov et al. (2002), Wu et al. (2004), and Zhang et al. (2001).

Generally, XML labeling schemes can be categorized into two groups: static labeling schemes (Amagasa et al., 2003, Li and Moon, 2001, Tatarinov et al., 2002, Zhang et al., 2001) and dynamic labeling schemes (Cohen et al., 2002, Li and Ling, 2005, Li et al., 2006, Li et al., 2008, Min et al., 2007, Min et al., 2009, O’Neil et al., 2004, Wu et al., 2004). Static labeling schemes are adequate when the XML documents are not updatable while dynamic labeling schemes are more suitable when the XML documents can be updated. The advantage of using static labeling schemes is that they need small storage space. However, inserting a new node into the XML tree may involve re-labeling a large number of pre-existing nodes. In dynamic labeling schemes, re-labeling the pre-existing nodes is avoided or at least smaller than static labeling schemes but the length of labels increases dramatically when new nodes are inserted into the XML tree especially when new nodes are inserted frequently at a fix place.

The aim of this paper is to devise a novel encoding scheme to encode the labels of the XML nodes so that the need for re-labeling the pre-existing nodes can be avoided. To achieve this, we rely on fractional numbers. The key feature of fractional numbers is that infinite number of fractional numbers can be inserted between any two unequal fractional numbers. We believe that the problem of re-labeling the pre-existing nodes during the XML updating process can be solved if the XML nodes are encoded with the fractional numbers. Hence, the main contributions of this paper are summarized as follows:

• We devise a novel encoding scheme based on the fractional numbers to encode the labels of the XML nodes. With this new scheme, we are able to insert new nodes between two consecutive nodes without affecting the order of pre-existing nodes.

• We propose a mapping method to convert our proposed fractional number based encoding scheme to bit string based encoding scheme with the intention to minimize the label size and save the storage space.

• We apply our proposed bit string encoding scheme to the range-based labeling scheme and the prefix labeling scheme and perform several experiments to demonstrate the benefit of our proposed bit string encoding scheme for querying and updating the XML data.

• We devise an algorithm to control the label size increment of our proposed bit string encoding scheme when new nodes are inserted frequently at a fix place of an XML tree.

The remainder of the paper is organized as follows: in Section 2, existing XML labeling and encoding schemes are investigated. In Section 3, our proposed fractional number based encoding scheme is presented. In Section 4, we explain how to convert our proposed fractional number based encoding scheme to bit string encoding scheme in order to reduce the storage space for storing the labels. In Section 5, we apply our proposed bit string encoding scheme to the range-based labeling scheme and the prefix labeling scheme. In Section 6, the process of XML querying is described. In Section 7, the process of XML updating is described. In Section 8, the process of skewed insertions is explained. Experimental results are illustrated in Section 9. Finally, the paper is concluded in Section 10.