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problem with blank nodes in EXISTS solution mappings

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 16 Jun 2016 15:38:02 -0700
To: public-sparql-dev@w3.org
Message-ID: <de4891ae-f0ac-675c-22c7-7d1fb0c8c787@gmail.com>
The definition of EXISTS in SPARQL 1.1 Query uses the substitute function.
There are multiple problems with substitute already reported.  Here is
another one.


  :s :p ?x .
  FILTER EXISTS { ?x :p ?y . }

on the graph

:s :p _:x .

The EXISTS gets { { (x,_:x) } } and then does a substitute which results in
  BGP( _:x :p ?y )
This has a solution of
  { { (y,:x) } }
because the blank node that is the mapping of x can itself be further mapped
to :s in the RDF instance mapping part of a pattern instance mapping.

So the result of the SELECT is
  { { ( x, _:x ) } }
which is not at all what should be expected.

This seems to be a severe problem for the substitution definition of EXISTS.

I suggest that the substitution definition of EXISTS be replaced with a
definition that uses initial (multisets of) solution mappings.  This would
solve the problems with substitution into BIND, VALUES, and SELECT

Peter F. Patel-Schneider
Nuance Communications
Received on Thursday, 16 June 2016 22:38:32 UTC

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