Re: Issue-3 - Blank nodes substituted into BGPs act as variables

I don't see how an entailment regime can be used to good effect here.

Remember that the exists substitution replaces  variables with blank nodes.
These blank nodes are, to SPARQL, just like any blank node that was already in
the BGP so I don't see how blank nodes from the two different sources can be
treated differently, according to the SPARQL specification or according to an
entailment regime.

Of course, they *can* be treated differently.  It is just that there is no way
that this can be done using the machinery that is available in the SPARQL
specification.  A way of proceeding could be to add a new kind of thing to
BGPs - a protected blank node - that would not be subject to replacement.  I
don't think that this is the right way forward.

peter


On 07/14/2016 06:19 AM, Andy Seaborne wrote:
> For issue 3, what the spec says and what users expect are different.
> 
> Query:
> ---------------------
> PREFIX : <http://example/>
> 
> SELECT * {
>    ?s ?p ?o
>    FILTER EXISTS { ?s :p 1 }
> }
> ---------------------
> 
> Data:
> ---------------------
> PREFIX : <http://example/>
> 
> _:s1 :p 1 .
> _:s2 :p 2 .
> ---------------------
> 
> Outcome 1:
> -----------------
> | s    | p  | o |
> =================
> | _:s1 | :p | 1 |
> -----------------
> 
> I think this is expectation - the ?s in the EXISTS becomes the blank node and
> the EXIST test is about triples with the blank node from the BGP.
> 
> Compare this to when the data is:
> ---------------------
> PREFIX : <http://example/>
> 
> :s1 :p 1 .
> :s2 :p 2 .
> ---------------------
> giving:
> ----------------
> | s   | p  | o |
> ================
> | :s1 | :p | 1 |
> ----------------
> 
> The intuition is that data blank nodes and IRIs should behave the same.
> 
> But in the spec blank nodes behave like variables, not constants.
> 
> Spec gives:
> -----------------
> | s    | p  | o |
> =================
> | _:s1 | :p | 1 |
> | _:s2 | :p | 2 |
> -----------------
> 
> because it is like:
> 
> PREFIX : <http://example/>
> 
> SELECT * {
>    ?s ?p ?o
>    FILTER EXISTS { ?VAR :p 1 }
> }
> 
> and { ?VAR :p 1 } matches for any ?s ?p ?o.
> 
> The bnode _:s1 or _s2 substitute into the EXISTS giving  { [] :p 1 } for each
> row.
> 
> I'm not sure of the way to address this.
> 
> Peter - can an entailment regime impose a condition that blank nodes in the
> data only map to the same blank node?
> 
> Another approach is to somehow add a restriction on the "variableness"
> c.f. if it were a true variable:
> 
> SELECT * {
>   ?s ?p ?o
>   FILTER EXISTS { ?VAR :p 1 . FILTER(sameTerm(?VAR, ?s) }
> }
> 
> Any other ideas?
> 
>     Andy
>

Received on Thursday, 14 July 2016 14:03:15 UTC