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<html>
<head>
<title>TIMBER</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body bgcolor="#FFFFFF">
<table width="600" border="0" cellspacing="0" cellpadding="10" align="center" bordercolorlight="#FFFFFF" bordercolordark="#CCFFFF">
<tr align="center" valign="bottom" bgcolor="#6666FF">
<td colspan="2" height="100">
<div align="center">
<img src="timber2.gif" width="477" height="128">
</div>
</td>
</tr>
<tr>
<td width="100" valign="top" bgcolor="#CCFFCC">
<p> </p>
<p><a href="introduction.html">Introduction</a></p>
<p><a href="overview.html">Overview</a></p>
<p><a href="support.html">Support</a></p>
<p><a href="people.html">People</a></p>
<p><a href="publications.html">Publications</a></p>
<p><a href="download.php">Download</a></p>
<p><a href="usermanual/index.htm">User manual</a></p>
<p><a href="bug.php">Report bugs</a></p>
<p><a href="links.html">Useful links</a></p>
</td>
<td bgcolor="#CCCCFF" valign="top" align="left">
<h2>Overview</h2>
<p>The central idea in the Timber project is to construct an XML database
that has an architecture as close as possible to that of a relational
database. In this way, we attempt to reuse, where appropriate, the technologies
developed for relational databases over the past several decades. See
(<a href="files/timber.pdf">Timber overview</a>) for an overview of the
project. </p>
<p>Relational Algebra plays a central role in relational database implementation.
An equivalent algebra for XML is developed in <a href="files/tax_full.pdf">TAX</a>.
This algebra manipulates sets of ordered labeled trees, rather than sets
of tuples. The primary difficulties addressed include the complex and
variable structure of trees in a set, and issues of ordering. </p>
<p>Once a query has been parsed into an algebraic representation, this algebraic
expression is manipulated in a query optimizer, which then chooses an
appropriate query plan to evaluate the query. The (<a href="files/optimization.pdf">Query
optimization for XML</a>) points out some key differences for XML query
otimization vis-a-vis relational optimization. </p>
<p>Cost-based query optimization requires good size estimates for query
results and intermediate results. Due to the complex structure of XML
data, obtaining these estimates is not necessarily straightforward. Techniques
for obtaining such estimates effectively are discussed in (<a href="files/yEDBT02.pdf">Estimating
answer sizes for XML queries</a> and <a href="files/twig-icde2001.pdf">Counting
Twig Matches in a Tree</a>). </p>
<p>XML query evaluation often involves the computation of a structural join.
One has to determine the satisfaction of predicates at multiple structurally
related elements in the database. New access methods for the computation
of such joins are introduced in (<a href="files/sICDE02.pdf">Structural
Joins</a> and <a href="files/comboperators.pdf">Combining Operators in
XML Query Processing</a>). </p>
<hr>
<p align="right"><img src="email.gif" /><i><font size="2"><br>Page last modified on 09/May/06</font></i></p>
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</body>
</html>