Furthermore, data warehouses can be used to store XML documents and WWW
data. A data warehouse storing information represented by means of XML is called
XML data warehouse (Marian, Abiteboul, Cobena, & Mignet, 2001), and a data
warehouse collecting information from the Web is called Web data warehouse
(Bhowmick, Madria, Ng, & Lim, 1998). In the literature are also considered XML
Web data warehouses (Marian et al., 2001; Wang & Zaniolo, 2003).
A dynamic warehouse for XML data was proposed and implemented in the Xyleme
project (Xyleme, 2001). The prototype was then turned into a product by a startup
company also called Xyleme.
Information stored into a data warehouse is often time varying, thus as for semistructured
data, also in the data warehouse context, it could be useful to consider
time. The classical time dimensions, considered in the literature, are transaction
time and valid time. The transaction time is the time when a fact is current in the
database and may be retrieved, while the valid time is the time when a fact is true
in the considered domain (Jensen, Dyreson, Bohlen, et al., 1998).
In the semistructured data context, graph-based data models have been extended to
represent the time dimension of information, and issues related to the representation
of transaction and valid times have been studied (Chawathe, Abiteboul, & Widom,
1998; Combi, Oliboni, & Quintarelli 2004; Oliboni, Quintarelli, & Tanca, 2001).
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