Database Reference
In-Depth Information
Computing Compressed XML Data from
Relational Databases
Stefan Böttcher, Dennis Bokermann, and Rita Hartel
University of Paderborn, Computer Science, Fürstenallee 11, 33102 Paderborn, Germany
{stb@,dbn@mail.,rst@}uni-paderborn.de
Abstract.
SQL/XML allows generating an XML document as the result of a
query that is evaluated on relational data. This facilitates companies sharing
their relational data in form of XML documents with other companies or with a
marketplace, but significantly increases the exchanged data volume. To reduce
both the volume of the exchanged data by exchanging compressed XML and
the time needed for compression, we propose an approach that allows prepara-
tion of a compressed XML document as the answer to an SQL/XML query di-
rectly, i.e., without the need to create the XML document first and compress it
afterwards. Our evaluation has shown that generating the compressed document
directly is in most cases faster than generating the uncompressed XML docu-
ment and compressing it, and in some cases it is even faster than the generation
of the uncompressed XML document alone. As our approach of generating
compressed XML requires only SQL support from the underlying database sys-
tem, a second advantage is that it can be used for the generation of compressed
XML even for database systems that do not (yet) support SQL/XML (like
MySQL).
Keywords:
XML compression, relational Databases, SQL/XML.
1 Introduction
1.1 Motivation
Since SQL:2003, SQL is extended by SQL/XML to allow use of XML in conjunction
with SQL. The SQL/XML specification includes functions to construct XML data.
These functions allow the user to construct new XML elements or attributes with text
values or attribute values taken for example from relational tables. Other functions
such as XMLCONCAT or XMLAGG can be used for combining small XML frag-
ments into larger ones. Therefore, SQL/XML makes it possible to create XML docu-
ments as the result of a query which is evaluated on a relational database.
Typical applications of SQL/XML are cross-company applications where compa-
nies transform parts of their relational data into XML and send the transformed XML
data to a second company, to a customer or to a marketplace. While the XML format
due to its flexibility is a popular data format, the overhead caused by this flexible
structure is the biggest disadvantage of using XML as data exchange format. Ex-
changing a compressed XML representation instead of uncompressed XML might
