ON XML QUERY "ALGEBRA"

From: Ken North

Date: 1 Nov 2005

In his writing, Fabian Pascal has cited comments you've made about the XML query algebra. That peaked my curiosity but I'm unable to find anything you've written about the subject.

THE THIRD MANIFESTO was published before these W3C documents:

1. XML Query Algebra published in 2001 (superseded)

2. XQuery 1.0 and XPath 2.0 Formal Semantics (Sept. 2005)

Have you published any point-by-point analysis of the earlier query algebra or the more recent XQuery semantics documents?

From: Fabian Pascal

Now, my eyes light up at the word "algebra" ... Originally, I understood it to mean a set of operations that are closed over some type. That is, every operation in X Algebra operates on zero or more values of type X and returns a value of type X. Hence, set algebra, Boolean algebra, relational algebra and the algebra of numbers that gives us arithmetic. Over what is the XML Query Algebra closed? Nobody has ever given me an answer that makes sense (apart from the occasional, honest "I don't know").

--Hugh Darwen

From: Hugh Darwen

Yes, I remember writing that, but I'm not sure when. FYI, what I wrote then remains true today, but only because I stopped asking and haven't had any reason to spend time on further investigation. The people I had asked up to that point included certain members (at the time) of the W3C committee developing XQuery.

From a brief look at the September 2005 document, I can guess that every XQuery expression perhaps operates on one (zero?) or more sequences of zero or more things each of which is either an atomic value, or an element, or an attribute, or a document, or a text, or a comment, or a processing-instruction node, yielding one such sequence. I am already struck by the complexity, without delving into what any of these things might be.

From: Fabian Pascal

It is rather revealing that the very raison d'etre of XML idea—the document—had to be discarded in favor of the "sequence" abstraction, which says about everything you need to know about the whole endeavor.

That complexity is what Codd was smart enough to avoid with RM. Those who don't know, forget, or ignore the past are doomed to repeat it.

From: Hugh Darwen

Absolutely!

Posted 1/27/06