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Use Case: Cheapest Flight

From: Eric Prud'hommeaux <eric@w3.org>
Date: Thu, 1 Apr 2004 13:54:10 -0500
To: public-rdf-dawg@w3.org
Message-ID: <20040401185409.GG15738@w3.org>

While we're contemplating negation (safe, stratified, mystified), we
should also deal with the practical problem of aggregate functions.

Find me the cheapest flight (in the database) from Boston to Chicago.

The merits of the use caes are pretty obvious, it's the sort of
problem we solve whenever we select from amongst a finite set of
options. It's also pretty contentious as it specifically closes the
world -- it causes the server to provide answers that it may retract
if it encounters more data.

My two cents:
The semantics of the query language don't have to follow the semantics
of the data it queries. It can allow the user to tacitly assert a
premise like "...that you know of right now". People intuitively grasp
this constraint and check multiple sources for things like cheap

A couple more cents:

An agent that is processing a portion of an RDF document (eg FOAF
scutterers looking for some FOAF and DC arcs and ingoring the rest)
must be confident that the arcs that it *doesn't* interpret don't
contradict its interpretation of the arcs it *does* interpret. Hence
you don't come along after the fact and change meaning of a foaf:knows
arc by adding arc saying "well, doesn't know yet, but will meet soon".
That would break the interpretion of foaf:knows that everyone had
programmed into their heads and their agents.

I don't think we have such goals for queries. A query caching
mechanism could, by the same analogy, be hampered by the results to
queries changing as the knowlege source gets more information. And
certainly the querier will be expected to publish the results
reponsibly (and not tell the world that for time, UA836 is the
cheapest flight form Boston to Chicago). But I think this use case is
common enough that it may be more practical to allow aggregats and
stratified negation and make sone constraints about the interpretation
of the results.

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Received on Thursday, 1 April 2004 13:54:22 UTC

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