W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > October to December 2004

Re: apologies and regrets

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Wed, 08 Dec 2004 16:24:52 +0000
Message-ID: <41B72AD4.4010500@hp.com>
To: Pat Hayes <phayes@ihmc.us>
CC: RDF Data Access Working Group <public-rdf-dawg@w3.org>


Thank you for the comments and for the rewrite of the defintions.  It will take 
Eric and I awhile to process them.  There are some answers to questions in the 
email text below.


Pat Hayes wrote:
> First, Im sorry Im so far behind, and missed the last telecon. (I 
> thought I could attend through IRC, but Mark Greaves pulled the 
> network plug during the DAML meeting. ) Second, I have to send 
> regrets for tomorrow as well, unfortunately, and will make the next 
> week only on a cell phone with no network connection (I should be 
> back online by Wednesday of that week).
> I agreed to review the definitions in
> http://www.w3.org/TR/2004/WD-rdf-sparql-query-20041012/
> but its taking a lot longer than I thought it would partly because I 
> had a lot of trouble understanding the document. It seems to have a 
> lot of internal contradictions (??). Anyway, some of my problems and 
> comments and so on are visible in an annotated version at
> http://www.ihmc.us/users/phayes/SPARQL-PAT.html
> I started trying to edit the text (pink deletions and additions) but 
> eventually I had to give up, as I couldnt figure out exactly what it 
> meant. Hence the pink verdana-font comments. Sorry if these have a 
> rather crotchety tone, they are surface-of-consciousness reactions to 
> my frustration at trying to figure out the meaning. Having gone 
> through the document twice I think I now have the general picture 
> rather clearer; but if Im right, then I think the document needs a 
> major re-write, as in its current form it keeps making definitions 
> which it then almost immediately contradicts. Eg a pattern is not a 
> set, in general.
> Anyway, Ive drafted a more positive summary of the way that the 
> definitions and constructions might go.
> http://www.ihmc.us/users/phayes/SPARQLdefs.html
> This is bare definitions, minimal comments, no examples. Stuff in 
> italics is comment rather than content.
> As per the verdana comments, there seems to be no sense in allowing 
> patterns to have bnodes and distinguishing a subset of variables, 
> since a pattern variable which is not selected plays exactly the same 
> role as a bnode in the pattern would play. Im assuming therefore that 
> patterns do not contain bnodes. If this is wrong its easy to tweak 
> the definitions, but it only adds complexity and provides no extra 
> expressiveness.
> General issues that emerged include:
> 1. Do we want to allow query variables to be placed on parts of 
> literals, eg querying the type of a literal or asking for literals of 
> a certain type? Or looking for languages:
> SELECT ?v WHERE (?x ?p "cat"@?v)
> (Why not?)

I hope so but it hasn't been agreed.

> 2. What is an answer to a query with a selected variable which does 
> not occur in the pattern?

The variable is not bound in any query solution.

Jos proposes that a triple template (the things in the CONSTRUCT clause) are 
skipped if they involeve an unbound variable but other triple templates are 
still done in the same query solution.

> 3. What are the rules for answering a limited-number-of-answers 
> query? (do we get the NEXT n answers if we repeat the query?)

Best effort.  LIMIT is there for pragmatic reasons, like wanting to show the 
user a sample of what the (unlimited) query finds or just to put a network limit 
in case of unexpected large results.

And, no, you don't get the NEXT n on a repeat.  There is no state across requests.

Asking again may, or may not, get the same N results.  Nothing is fixed.

> 4. . Are constraints intended to be normative? The text in 3.1 seems 
> to say not, but then why does the document spend so much time on them?

There will be a number of normative constraints - the exact ones is still to be 

> 5. What does section 10.3 specify? As far as I can tell, it just says 
> that the answer can be some arbitrary piece of RDF. OK, but who 
> decides what is in that RDF?

The server (information publisher) decides.  A good implementation returns 
something useful and hence is used by applications.  The point is not to 
prescribe the results because the client does not know what's available.  It's 
an open question, where SELECT, CONSTRUCT are closed questions - the client is 
fixing the shape of the answers.

When I ask for a web page, there isn't a "correct" answer.
Extreme example: news sites chnage the amount of items in the HTML returned 
under conditions of high load.

> 6. There are several alternative forms for the answer to a SELECT 
> query. Who decides what form the answer will take? Can the query 
> specify this? How? If not, how can a querying app know how to process 
> the answer?

Content negotiation (or API equivalent - the app asks for the form it wants).

   application/xml => get the XML form.
   application/rdf+xml => get an RDF/XML form.
   turtle, N3, N-triples similarly as serializations of the RDF form.


> -------
> Pat
Received on Wednesday, 8 December 2004 16:26:10 UTC

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