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Re: Two question about your data model

From: Mary F. Fernandez <mff@research.att.com>
Date: Wed, 21 Jun 2000 13:58:54 -0400
Message-ID: <3951025E.1FDE9EDA@research.att.com>
To: sartiani@di.unipi.it
Cc: www-xml-query-comments@w3.org
Hi Carlo,
This message is in response to your message:
http://lists.w3.org/Archives/Public/www-xml-query-comments/2000May/0009.html


>
> Two question about your data model
>
> From: Carlo Sartiani (sartiani@di.unipi.it)
> Date: Wed, May 31 2000
>
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> Message-Id: <a04310100b55ab40b98ba@[131.114.2.83]>
> Date: Wed, 31 May 2000 14:34:49 +0200
> To: www-xml-query-comments@w3.org
> From: Carlo Sartiani <sartiani@di.unipi.it>
> Cc: sartiani@di.unipi.it
> Subject: Two question about your data model
>
> I read your working draft and I have two simple question:
> 1) A data model instance is defined as a forest of trees (maybe
> consisting of a single node). In Appendix B you defined two forms of
> ordering among forest nodes, highlighting  that:
>
> "Given a single input document, a global, total order can be defined
> on
>                                                    nodes. In general,
> however, it may not be possible to define a global order, for example
> when a data model instance contains nodesfrom multiple input
> documents."
>
> In Quilt definition paper (Sigmod/Pods/WebDB2000) authors, instead,
> explicitly stated that on every data model instance a total order
> relation is defined:
>
> "Each instance of the XML Query data model (regardless of whether it
> is a complete document, a fragment of a document, or a list of
> documents) is a forest that includes a total ordering, called
> "document order," among all its nodes."
>
> Are Quilt authors wrong or, instead, do you think to enrich XML Query
> Data Model with a mechanism for defining a total order relation among
> nodes coming from multiple input documents?
>

We cannot comment directly on the Quilt definition of global order
among nodes from multiple documents, but we do encourage you to
contact the authors of the Quilt paper for clarification.

The XML Query working group expects to revisit the issue of global
order when we define the XML Query Algebra and MAY define a global
order on an ordered collection of documents.

>
> 2)XML Query Data Model represents an XML document as a node-labelled
> tree, while traditional semistructured database data models exploit
> edge-labelled graphs (e.g., XML-QL). What are the main reasons behind
> your choice and what are the main advantages you think to obtain by
> using a tree representation instead of a graph representation?
> --
> Carlo Sartiani
> Ph.D. Student
> Computer Science Department
> University of Pisa
>
> "Here's to the crazy ones. The misfits. The rebels. The troublemakers.
> The round pegs in the square holes. The ones who see things differently.
> The ones that change the world!"
>
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The primary reason is to be compatible with existing XML data models,
in particular the XPath data model is a node-labeled, tree-constructor
model and the XML Information Set as exposed by the Document Object
Model API is the same.  The XML Query Data Model does support a
concept of node identity (the Ref(Node) value), which simplifies the
representation of XML reference values, e.g., IDREF, XPointer, and URI
values.

Thank you,
Mary Fernandez
W3C XML Query Working Group.

--
Mary Fernandez                    AT&T Labs - Research
Principal Technical Staff Member  180 Park Ave., Bldg 103, E243
mff@research.att.com              Florham Park, NJ 07932-0971
http://www.research.att.com/~mff  973-360-8679 FAX: 973-360-8187
Received on Wednesday, 21 June 2000 13:59:01 UTC

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