Re: Fwd: StateOfTheArt Survey

Hi Sebastian,
My comments are inline:
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The field "preserving SPARQL semantics to SQL" is a very wide field.
Although there might be some insight gained from this, it doesn't seem 
worth the effort to get into detail there. Almost every Triple store has 
its own rewriting engine (RAP, Jena, Virtuoso). It can hardly be 
compared to Squirrel RDF or Relational.OWL or anything. I personally had 
difficulties in seeing the connection to the RDB2RDF issue.
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The idea is to describe the rewriting approaches of different implementations using the mechanism outlined in "relational algebra for SPARQL" [Cyganiak, 2005] as reference. Since this is a survey, we should not wade into the research issues of SPARQL to SQL rewriting.
 
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I'm not sure in which direction this criteria goes. Does 
federated/distributed mean querying over different databases or just 
different tables. There are some issues mentioned in "D2RQ lessons 
learned" [1] about this in 3.1 and 3.2. If it is concerned with 
distributed queries over several endpoints DARQ[2] should be considered.
<snip>
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Thanks for pointing to DARQ, we will review it and add to the survey.
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1. Problem of Mapping page 4
Reasoning is just one of the nice features of OWL, but not the major 
advantage of RDF as such. The real advantage is the different knowledge 
representation paradigm and the ability to model additional knowledge 
and query the graph with SPARQL. Mark the famous DBpedia SPARQL query “A 
soccer player with #11 shirt in a club with a stadium of over 40,000 
seats born in a country with over 10 M inhabitants” which returns 10 
players. DBpedia is in RDF-S, no OWL or reasoning used, but still the 
possibility to query it, can reveal a great deal of "knowledge" (data 
put into a certain context).
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I am not sure of the issue here - using RDF to integrate data also allows reasoning (with/without an associated schema/model that is defined in RDFS/OWL). There is no RDF(S) vs OWL debate in this section.  
 
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Components of Survey Framework p.5
1. Mapping approach
The ER diagram, although being close to RDF from a conceptual viewpoint, 
doesn't have expressed semantics unlike a relational database. It is 
primarily a diagram. I'm not up to date in the latest trends in db 
modelling, but rel. database change over time according to the 
application needs, e.g. performance, new values and I'm not sure, if 
there is an ER diagram for a matured database any more. Maybe somebody 
else knows if something like ER diagram semantics exists and are 
reproducable from a db. I would be interested in that.
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We are referring to the Entity-relationship model as a db modeling method. Please refer to "Relational Databases on the Semantic Web", T. Berners-Lee, 1998 for discussions on ERD and RDF.
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2. Mapping Representation and access:
substitute: mapping algorithm by paradigm or design or just: The mapping 
used for conversion of...
it should be made more clear that "Access" in the title doesn't mean how 
the data is accessed, but how 
accessible/understandable/shareable/modular the mapping definition is
( at least that is how I understood it)
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modified.
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4. Mapping Implementation p.6
There might not be a performance penalty. See [4] RDF Views is faster 
than the triple store. The table from Barrasas slides [5] page 4 could 
be copy/pasted here as it gives a good overview. I'm currently not so 
sure, what a disadvantage of on-demand querying could be. Maybe that 
there is no update, write-back process, e.g. SPARUL or so.
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Barrasas slides link http://www2006.org/programme/files/pdf/p160-slides.pdf does not work. Is there an alternate reference/link?
 
Cheers,
Satya Sahoo
http://knoesis.wright.edu/researchers/satya