From: Matthew Perry <matthew.perry@oracle.com>

Date: Tue, 25 Jan 2011 12:56:57 -0500

Message-ID: <4D3F0EE9.6050303@oracle.com>

To: Andy Seaborne <andy.seaborne@epimorphics.com>

CC: W3C SPARQL Working Group <public-rdf-dawg@w3.org>

Date: Tue, 25 Jan 2011 12:56:57 -0500

Message-ID: <4D3F0EE9.6050303@oracle.com>

To: Andy Seaborne <andy.seaborne@epimorphics.com>

CC: W3C SPARQL Working Group <public-rdf-dawg@w3.org>

Hi Andy, Thanks for the explanation. I completely understand and agree with the cycles in fixed-length property paths. My concern is with the arbitrary-length paths. I would strongly prefer to have a purely node-marking algorithm instead of triple marking. The pure node marking seems to better match the resolution we passed back in June: http://www.w3.org/2009/sparql/meeting/2010-06-01#resolution_5. (The two solutions with cycles both repeat nodes). PROPOSED: The cardinality of solutions to fixed-length paths is the same as the cardinality of solutions to the path expanded into triple patterns (with all variables projected);*the cardinality of solutions to variable-length paths is the cardinality of solutions via paths that do not repeat nodes;* the cardinality of solutions to paths combining fixed and variable length (elt{n,} ) is a combination of the fixed definition plus the variable definition for paths longer than the fixed length. Thanks, Matt On 1/25/2011 12:02 PM, Andy Seaborne wrote: > > > On 25/01/11 14:41, Matthew Perry wrote: >> For pp16 and pp25, these paths repeat a node (i.e. contain a cycle). I >> thought we were using a node marking algorithm to prevent cycles. > > We currently have a triple marking algorithm which puts in the end node which means you can get a duplicate from a cycle. > > We could change ArbitraryLengthPath to also record node passed through. > > For me, a consistent view is more important. Also, with cycles > > Data: (DAG, not cycle). > > :a :p :b . > :b :p :z . > > :a :p :c . > :c :p :z . > > and > > { :a :p/:p ?X } > { :a :p ?v .?v :p ?X } # SPARQL 1.0 > > have the endpoint twice. > > { :a :p* ?X } has :z twice. > > { :a :p/:p ?X } > > Data: cycle: > > :a :q :b . > :b :q :c . > :c :q :a . > > and { :a :q+ ?X } > does not have a duplicate but :q* does because it includes the starting point and :q* = :q{0} UNION :q+ > > > > 2) pp16 -- I don't understand why the path d --> e --> f --> e is included. > > pp16 is a bit unnatural if read with interpretation of the meaning of the property: find me everyone ?X knows, recursively is "+", not "*" > > Data contains: > > :d foaf:knows :e . > :e foaf:knows :f . > :f foaf:knows :e . > > { ?X foaf:knows* ?Y } = > { ?X foaf:knows{0} ?Y } UNION { ?X foaf:knows+ ?Y } > > { ?X foaf:knows{0} ?Y } ==> ?X=:d, ?Y=:d > { ?X foaf:knows+ ?Y } ==> ?X=:d, ?Y = :e, ?f then :e again as > the triple ":f foaf:knows :e" is travered, the endpoint included and the cycle stops. > > Any loop where ArbitraryLengthPath is not starting in the cycle is going to give a duplicate and it differenates from the same without the final step (no cycle). > > > 3) pp25 -- I don't understand why the path a --> c --> c is included. > > :a :p :c . > :c :p :c . > > so :c goes to :c (a cycle of one triple). > > AndyReceived on Tuesday, 25 January 2011 17:57:45 GMT

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