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Re: ACTION item(s): definition(s) and an algorithm for postprocessing minimization

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Mon, 21 Aug 2006 11:00:43 +0100
Message-Id: <EDFE6C8C-831B-4684-AB9E-D2755227EE3D@cs.man.ac.uk>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: andy.seaborne@hp.com

On Aug 21, 2006, at 10:53 AM, Seaborne, Andy wrote:

> Bijan Parsia wrote:
>> I believe that the qualms Andy had about what happens in  
>> intermediate  steps was do the different interpretations of what  
>> the minimized  final answer set would look like. That is, his  
>> understanding was  close to 1 (like pat) and mine were entirely  
>> different. However,  algorithms for computing 2 and 3 are easy  
>> enough, so I'm doing the  action item anyway.
>
> My reservations are:
> 0/ BNodes over the graph matter - not answer sets
> 1/ Lack of streaming
> 2/ Additional processing cost [*]
> 3/ Non-standard processing (i.e. can't be, say SQL DISTINCT).

Thanks for the clarifications.

> [*] Even assuming you want to do it at all, graphs can be leaned  
> once, then queried many times.  Answer sets have to be leaned on  
> every query result.

Very true. Though if we deal with updates as many real engines do,  
the gap narrows a bit. Also, for many applications (cmd line based  
query where you load the file each time) this is not true. Of course,  
it is most true where you need the most speed, i.e., servers with big  
triples store and lots of queries. OTOH, it's unclear how painful  
typical queries will be. A lot depends on the size of the result set  
and the amount of redundancy.

>> So, first qualification: These algorithms are only minimizing  
>> with  respect to BNodes. You have to plug in your own account of  
>> literals.  I use "row" for "answer" avoid confusion:
>> DEFINITION 1: Answer graph template
>> 	Let A be an answer set and Avar be the set of column headings of A.
>> 	The answer graph template of A is the set of triple patterns,  
>> such  that:
>> 		{tp | _:row ('http://var.org/#" ++ var) var. & var \in Avar}
>                                                         ^^^
> value?

I just needed a random URI prefix. Substitute any you like.

> (Just to show I'm reading this all!)

Heh.

Oh, if we had (do we have?) CONSTRUCT DISTINCT, I would personally  
expect a lean graph as output.

Cheers,
Bijan.
Received on Monday, 21 August 2006 10:00:55 GMT

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