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Re: can subqueries be executed first in SPARQL?

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Fri, 17 Jun 2016 05:10:52 -0700
To: Axel Polleres <droxel@gmail.com>, james anderson <james@dydra.com>
Cc: public-sparql-dev@w3.org
Message-ID: <f8aecedf-5ef6-3cd8-a7e6-f3eda8b08376@gmail.com>
On 06/17/2016 12:52 AM, Axel Polleres wrote:
> Hi all,
> 
> interesting discussion...
> 
> First, let me say that I find the claim that something is "broken" here exaggerated.
> EXISTS is defined in a semantically unambiguous way, it is just what it is.

Well, aside from the interesting cases, the substitutions can end up replacing
variable arguments to Project and Extend and Graph and even the variables in
the multisets that result from the translation of inline data   with RDF
terms.  This generally results in expressions whose meaning is unspecified or
are illegal.  This is broken in a very strict sense of broken.

> The corner case about blank nodes is still quite interesting.
> 
> On a side note, we had proposed a solution for this problem in XSPARQL [1,2]
> where we treated the problem of matching blank nodes in nested queries (cf. page 12 of [1]) in a manner that might rather reflect what you're looking for.
> 
> best regards,
> Axel
> 
> 
> 1. Stefan Bischof, Stefan Decker, Thomas Krennwallner, Nuno Lopes, Axel Polleres:
> Mapping between RDF and XML with XSPARQL. J. Data Semantics 1(3): 147-185 (2012) 
> http://link.springer.com/article/10.1007%2Fs13740-012-0008-7
> 
> 2. http://xsparql.sourceforge.net/
> --
> url: http://www.polleres.net/  twitter: @AxelPolleres

[...]

I took a quick look at this.  XSPARQL appears to treat blank nodes in BGPs as
if they were constants.  This is quite different behaviour from how they are
treated in SPARQL.

peter
Received on Friday, 17 June 2016 12:11:29 UTC

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