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Re: Negation decision : unexpected effects

From: Lee Feigenbaum <lee@thefigtrees.net>
Date: Sun, 04 Apr 2010 22:53:46 -0400
Message-ID: <4BB950BA.2000906@thefigtrees.net>
To: Andy Seaborne <andy.seaborne@talis.com>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>
On 4/2/2010 5:05 PM, Andy Seaborne wrote:
> The decision at F2F3 to have just the form of NOT EXISTs in an explicit
> FILTER has a limitation. I should have realised at the time but it
> didn't occur to me until after the meeting.
>
> FILTERs get moved to the end of the BGP during translation from syntax
> to algebra. The form without the word "FILTER" does not move e.g.
>
> { ?s rdf:type :T
> NOT EXISTS { ?s :p ?v . }
> ?s :q ?v
> }
>
> then NOT EXISTS is not moved about by the FILTER placement rules.

Yes, this is a difference. In most cases it doesn't matter, though, 
right? I'd like to understand better the cases in which moving/not moving
the NOT EXISTS changes the answers.

> One reason for NOT EXISTS is as a better form of the OPTIONAL/!BOUND
> idiom, where the OPTIONAL block anchors the location of the existence
> test by it's effect of setting, or not, a variable to be tested later.
>
> The FILTER could be artificially forced using a extra {}
>
> { { ?s rdf:type :T
> FILTER NOT EXISTS { ?s :p ?v . }
> }
> ?s :q ?v
> }
>
> This is unnecessary - the more direct syntax is closer to the
> OPTIONAL/!BOUND idiom that I think it the important thing to make easier.
>
> Therefore I propose we adopt both negation proposals without modification.

I wouldn't be in favor of this proposal - I'd find it very difficult to 
justify adding 2 forms of negation to SPARQL that seem virtually 
indistinguishable from one another in most scenarios.

> It only affects the grammar and translation from syntax to algebra -
> actual execution is unchanged and there is no additional implementation
> cost for execution.

To me, it makes sense to have one filter form and one graph pattern 
form. So far, I can't really make myself comfortable with the idea of 
adding two new graph pattern operators to the language that both do the 
same thing in almost all cases.

Lee

> Andy
>
>
>
Received on Monday, 5 April 2010 02:54:25 GMT

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