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Re: More on MINUS vs. UNSAID

From: Ivan Herman <ivan@w3.org>
Date: Mon, 13 Jul 2009 15:14:14 +0200
Message-ID: <4A5B3326.4030605@w3.org>
To: Gregory Williams <greg@evilfunhouse.com>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>


Gregory Williams wrote:
> On Jul 8, 2009, at 1:13 AM, Lee Feigenbaum wrote:
> 
>> Greg (& someone else?) explained UNSAID today as AntiOptional. I'm
>> wondering if "AntiOptional" is actually the same as
>> MINUS-AntiJoin+Restriction? Can anyone tell?
> 
> My (somewhat in jest) support for "AntiOptional" wasn't intended as a
> serious way to think about what UNSAID does. I liked it mostly because
> internally, my implementation of UNSAID differs from my implementation
> of OPTIONAL by only about 5 lines of code. The better way to think about
> it is, as Andy suggests, a filter.
> 
> I believe UNSAID is not the same as MINUS-AntiJoin+Restriction. Firstly,
> it seems that the Restriction is different, as UNSAID doesn't require
> intersection of the domains.
> 
> Second, I think another difference is going to appear in cases that
> don't make a lot of sense for "real" queries, such as using OPTIONAL
> within an UNSAID/MINUS block. Within an UNSAID block, this should have
> no affect, since OPTIONAL won't affect whether the pattern returns zero
> or some results. However, my (albeit sketch) understanding of
> MINUS-AntiJoin+Restriction is that an OPTIONAL within the MINUS could
> affect the compatibility of the RHS and LHS, if the OPTIONAL was unsafe.
> 
> In general, while it seems MINUS might be easier to formulate in
> algebraic terms (see Paul's email), I find that thinking about
> UNSAID/NOT EXISTS as a filter is conceptually much simpler than the
> various formulations of MINUS we've discussed (though this is clearly a
> personal preference).
> 

Having gone through the various emails I must say I agree (for now,
maybe somebody may convince me otherwise:-).

One reason for the issue of intuitiveness might be (in my case) that I
actually do not 'speak' sql. My mental model is in terms of triples
which seems to fit well with the NOT EXIST filter approach and thinking
in terms of solution tables needs an extra step... But that may be only me.

Ivan

> .greg
> 
> 

-- 

Ivan Herman, W3C Semantic Web Activity Lead
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Received on Monday, 13 July 2009 13:14:47 GMT

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