W3C home > Mailing lists > Public > public-rdf-wg@w3.org > December 2011

Re: dataset semantics

From: Sandro Hawke <sandro@w3.org>
Date: Sat, 17 Dec 2011 08:09:44 -0500
To: William Waites <wwaites@tardis.ed.ac.uk>
Cc: phayes@ihmc.us, david@3roundstones.com, public-rdf-wg@w3.org
Message-ID: <1324127384.6252.1425.camel@waldron>
On Sat, 2011-12-17 at 10:29 +0000, William Waites wrote:
> On Sat, 17 Dec 2011 00:43:38 -0500, Sandro Hawke <sandro@w3.org> said:
>     sandro> We haven't quite figured that out yet.  I'm proposing one
>     sandro> part of that is that a dataset being true implies its
>     sandro> default graph is true.
> What about this,
> :a {
>     :moon :substance :green_cheese .
> }
> :b {
>     :moon :substance :blue_cheese .
> }
> :c {
>     :bob :thinks :a .
>     :alice :thinks :b .
> }
> This is all fine and true and everything. But what then in the
> (common) case where the default graph is the union of all named
> graphs, we have the merge of :a and :b which (suppose) is false
> because we know the moon is only made of one kind of cheese?

Merging the named graphs may be common, but I don't think it's
truth-preserving.    So, don't merge the labeled graphs unless you're
willing to take responsibility for the conclusions.

In terms of an entailment test:

    <a>  { <b> <c> <d> }

does NOT entail

   { <b> <c> <d> }

Speaking of these default-is-the-merge datasets... 
I don't know how one is supposed to backup/restore them, or how one is
supposed to put metadata in them.   It would probably be good to have a
flag saying this is one of those default-is-the-merge datasets, but I
don't know where to put that flag, except in the default graph.   (In
SPARQL 1.1, I could put it in the Service Description, but that's not
part of the dataset, and wouldn't come out in a TriG file.)

Maybe default-is-the-merge should be handled as a special case: in
serial form, the default graph is { <> a rdf:DefaultIsMergedDataset },
but in a running server, that flag is elsewhere, and the default graph
is the merged group.    That's a total hack, though, and I'm sure will
cause other problems down the road.

    -- Sandro
Received on Saturday, 17 December 2011 13:09:59 UTC

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