Mustafa Atay

Efficient schema-based XML-to-Relational data mapping

Storing and querying XML documents using a RDBMS is a challenging problem since one needs to resolve the conflict between the hierarchical, ordered nature of the XML data model and the flat, unordered nature of the relational data model. This conflict can be resolved by the following XML-to-Relational mappings: schema mapping, data mapping and query mapping. In this paper, we propose: (i) a lossless schema mapping algorithm to generate a database schema from a DTD, which makes several improvements over existing algorithms, (ii) two linear data mapping algorithms based on DOM and SAX, respectively, to map ordered XML data to relational data. To our best knowledge, there is no published linear schema-based data mapping algorithm for mapping ordered XML data to relational data. Experimental results are presented to show that our algorithms are efficient and scalable.

A New Inlining Algorithm for Mapping XML DTDs to Relational Schemas

XML is rapidly emerging on the World Wide Web as a standard for representing and exchanging data. It is critical to have efficient mechanisms to store and query XML documents to exploit the full power of this new technology. While one approach is to develop native XML repositories that support XML data models and query languages directly, the other approach is to take advantage of the mature technologies that are provided by current relational or object-relational DBMSs. There is active research along both approaches and it is still not clear which one is better than the other. We continue our effort on the second approach. In particular, we have developed an efficient algorithm which takes an XML DTD as input and produces a relational schema as output for storing and querying XML documents conforming to the input DTD. Our algorithm features several significant improvements over the shared-inlining algorithm including overcoming its incompleteness, eliminating redundancies caused by shared elements, performing optimizations and enhancing efficiency.

On the consistency of XML DTDs

DTD has been widely used as the schema language for XML documents. A DTD describes the structure of a collection of similar XML documents. The consistency problem of XML DTDs concerns the question that given a DTD D, if there exists any finite XML document that conforms to D. This issue is important because one wants to know whether a DTD specification is meaningful. In this paper, we formalize the notion of the consistency of DTDs, identify a sufficient and necessary condition for a DTD to be consistent, and propose a linear algorithm, DTDCon, for the consistency checking problem.

Reconstructing XML Subtrees from Relational Storage of XML documents

Numerous researchers have proposed to use relational databases to store and query XML documents. One important component of such systems is the XML subtree reconstruction, which reconstructs the subtrees rooted at the matching nodes of an XML query and returns them to the user as the query result. Existing reconstruction algorithms either do not support recursive XML view schema, or require expensive nested queries or joins of multiple relations. In this paper, we propose an efficient XML subtree reconstruction algorithm, Reconstruct, which overcomes these limitations and uses an efficient stack-based structural join algorithm to recover all the parent-child relationships between elements. One salient advantage of this algorithm is that it employs the inlining feature of the inlining-based storage of XML documents, which is known as one of the best relational XML storage schemes. Both our algorithmic analysis and experimental study show that Reconstruct is efficient and scalable.

XML subtree reconstruction from relational storage of XML documents

Numerous researchers have proposed to use relational databases to store and query XML documents. In these systems, the elements selected by an XML query are returned to an application either by select mode or by reconstruct mode. For the reconstruct mode, the XML subtrees that are rooted at the selected elements need to be extracted and reconstructed from the relational storage of XML documents. Therefore, XML subtree reconstruction is an important problem since its efficiency has a significant impact on XML query response time. In this paper, we propose (i) a linear XML subtree reconstruction algorithm Reconstruct to reconstruct an XML subtree from the structure-encoded sequence of the subtree that is extracted from the relational database by a structure-encoded sequence retrieval algorithm, (ii) a generic efficient structure-encoded sequence retrieval algorithm RD-SB for a schema-based relational XML storage, and (iii) a generic efficient structure-encoded sequence retrieval algorithm RD-SL for a schema-less relational XML storage. To the best of our knowledge, our algorithms provide the first generic solutions to the XML subtree reconstruction problem that are applicable to all relational XML storage schemes proposed in the literature. Finally, our experiments show that our algorithms are efficient and scalable.

An efficient access control model for schema-based relational storage of XML documents

Very few research works have been done on XML access control over relational databases despite the fact that there is an undeniable amount of XML data stored in RDBMS. Moreover, the proposed algorithms in the literature have performance drawbacks. In this paper, we propose an efficient and well-defined XML access control model for schema-based relational storage of XML documents which overcomes the shortcomings of existing work. We also present the challenges of translating XML access control rules to relational access control rules and define an efficient and sound algorithm for this task.

Relational nested optional join for efficient semantic web query processing

Increasing amount of RDF data on the Web drives the need for its efficient and effective management. In this light, numerous researchers have proposed to use RDBMSs to store and query RDF annotations using the SQL and SPARQL query languages. The first few attempts at SPARQL-to-SQL translation revealed non-trivial challenges related to correctness and efficiency of such translation in the presence of nested optional graph patterns. In this paper, we propose to extend relational databases with a novel relational operator, nested optional join (NOJ), that is more efficient than left outer join in processing nested optional graph patterns. We design three efficient algorithms to implement the new operator in relational databases: (1) nested-loops NOJ algorithm, NL-NOJ, (2) sort-merge NOJ algorithm, SM-NOJ, and (3) simple hash NOJ algorithm, SH-NOJ. Based on a real life RDF dataset, we demonstrate the efficiency of our algorithms by comparing them with the corresponding left outer join implementations.

XML-to-SQL query mapping in the presence of multi-valued schema mappings and recursive XML schemas

Several query mapping algorithms have been proposed to translate XML queries into SQL queries for a schema-based relational XML storage. However, existing query mapping algorithms only support single-valued mapping schemes, in which each XML element type is mapped to exactly one relation, and do not support multivalued mapping schemes, in which each XML element type can be mapped to multiple relations. In this paper, we propose a generic query mapping algorithm, ID-XMLToSQL, for a schema-based relational XML storage. To the best of our knowledge, our algorithm provides the first generic solution to the XML-to-Relational query mapping problem that is applicable to both single-valued and multi-valued mapping schemes. Moreover, our algorithm also provides an elegant solution to the query mapping problem in the presence of recursive XML schemas and recursive queries. While existing algorithms need special recursion operators, our algorithm only requires the traditional relational operators and thus, can work with all relational databases.

A sufficient and necessary condition for the consistency of XML DTDs

Recently, XML has emerged as a standard for representing and exchanging data on the World Wide Web. As a result, there is a trend of increasing amount of XML documents that publish information on the Web from various data sources. A Document Type Definition (DTD) describes the structure of a set of similar XML documents and serves as the schema for XML documents. The World Wide Web Consortium has defined the grammar for specifying DTDs; however, even a syntactically correct DTD might be inconsistent in the sense that there exist no XML documents conforming to the structure imposed by the DTD. In this paper, we formalize the notion of the consistency of DTDs, and identify a sufficient and necessary condition for a DTD to be consistent.

An Experimental Study on Query Processing Efficiency of Native-XML and XML-Enabled Database Systems

With XML becoming a standard for representing semi-structured documents on the web and a standard for data exchange between different systems, some database companies are adding XML support to their existing database systems, while some other companies coming out with pure or native database systems for XML. In this paper, we present an experimental study on the query processing efficiency of a native-XML database system and an XML-enabled database system on a selected set of queries including operations from text-processing, DML and relation algebra. The experiments are conducted on two well-known commercial database systems using the web interfaces based on HTTP. The cost metrics we used are CPU time, the numbers of physical and logical reads. The queries were run on identical machines for 3 different sizes of documents with and without indexing. A subset of experimental results is presented and overall results are discussed. Generally speaking the XML-enabled system performed better.