[Copying Andy Seaborne and Steve Harris for their input.]
Hi Axel,
The part of the spec that I was trying to bring to your attention was
where it says that the empty group graph pattern "does not bind *any*
variables" (my emphasis). If ?G is not bound then that short form of
listing existing graphs would not work. But I am not sure from reading
the spec whether ?G is supposed to be bound or not. The SPARQL algebra
says:
http://www.w3.org/TR/sparql11-query/#defn_evalGraph
[[
eval(D(G), Graph(var,P)) =
Let R be the empty multiset
foreach IRI i in D
R := Union(R, Join( eval(D(D[i]), P) , Ω(?var->i) )
the result is R
]]
Hmm, there's right parenthesis missing, which I guess I'll report
separately.
It looks like Ω(?var->i) is binding the graph variable, but then it's
doing a "Join", which is defined as:
http://www.w3.org/TR/sparql11-query/#defn_algJoin
[[
Join(Ω1, Ω2) = { merge(μ1, μ2) | μ1 in Ω1and μ2 in Ω2, and μ1 and μ2 are
compatible }
]]
I couldn't readily find the definition of the merge function, but I
think I found the definition of "compatible":
http://www.w3.org/TR/sparql11-query/#defn_algCompatibleMapping
[[
Two solution mappings μ1 and μ2 are compatible if, for every variable v
in dom(μ1) and in dom(μ2), μ1(v) = μ2(v).
]]
which seems to be saying that the variables being joined must be bound
to the same values. But since the empty basic graph pattern does not
bind ?G, I *think* this means that Ω(?var->i) would *not* be compatible
and ?G would therefore not be bound in the result.
So, if I have properly followed the SPARQL algebra, I *think* this means
that the short form that you suggested for listing graphs will not work,
and it is not possible to get a list of graphs that includes empty
graphs. (Hence the Sesame 2.7.1 behavior is correct.)
Andy or Steve, have I got this right?
Thanks,
David
On 08/13/2013 03:10 PM, Axel Polleres wrote:
[[ The group pattern: { } matches any graph (including the empty
graph) with one solution that does not bind any variables. ]]
This only means that upon
SELECT ?G WHERE { GRAPH ?G {} }
also empty named graphs should be returned, which would not be the
case for
SELECT ?G WHERE { GRAPH ?G { ?S ?P ?O } }
Obviously, this makes a difference for all graph stores that support
empty named graphs. So, to my understanding at least, this is not a
bug in the spec.
HTH, Axel
On 13 Aug 2013, at 20:21, David Booth <[email protected]> wrote:
Hi Axel,
That doesn't work in Sesame 2.7.1 at least, apparently because ?G
is not bound, even though there is one solution. The SPARQL 1.1
spec says: http://www.w3.org/TR/sparql11-query/#emptyGroupPattern
[[ The group pattern: { } matches any graph (including the empty
graph) with one solution that does not bind any variables. ]]
Is this a bug in the spec?
David
On 08/13/2013 11:48 AM, Axel Polleres wrote:
Hi Quentin,
how about just
SELECT ?G WHERE { GRAPH ?G {} }
(no need to dump all triples, if the only concern is which ?G
exist)
BTW, [email protected] may be the list you wanted to use.
best, Axel
