draft response MK-1 (Fwd: Proposal for simplifying FILTER semantics)

I drafted a response for this

 http://www.w3.org/2009/sparql/wiki/CommentResponse:MK-1

Axel


Begin forwarded message:

> From: Markus Krötzsch <markus.kroetzsch@comlab.ox.ac.uk>
> Date: 2 May 2011 14:00:12 GMT+01:00
> To: "Polleres, Axel" <axel.polleres@deri.org>
> Subject: Re: Proposal for simplifying FILTER semantics
> 
> On 02/05/11 13:39, Axel Polleres wrote:
> >
> > On 2 May 2011, at 10:53, Markus Krötzsch wrote:
> >
> >> On 01/05/11 20:40, Polleres, Axel wrote:
> >>> OfflIist: warum funkt BIND fuer deinen use case nicht?
> >>
> >> Fuer FILTER "=" koennte das eventuell funktionieren (fuer andere
> >> Funktionen natuerlich nicht, aber da ist auch die praktische Motivation
> >> nicht so stark). Was mir da nicht so klar waere ist allerdings wie man
> >> BINDINGS disjunktiv mit (Sub-)Queries kombinieren wuerde. So wie ich die
> >> Semantik verstehe werden sie immer konjunktiv mit dem GroupGraphPattern
> >> verbunden. Man haette dann also wieder das gleich Problem, eine
> >> Disjunktion zwischen der Gleichheit mit den Bindings (FILTER) und
> >> etwaigen anderen Patterns (BGP) im WHERE-Teil auszudruecken.
> >
> > ja, aber das geht doch mit EXISTS im FILTER (auch ein neues feature in SPARQL1.1, dass genau deinen use case abzudecken scheint... nicht?)
> 
> Das habe ich noch nicht gefunden -- wo ist denn die Semantik von EXISTS
> definiert?
> 
> Markus
> 
> >>>
> >>> ----- Original Message -----
> >>> From: public-rdf-dawg-comments-request@w3.org<public-rdf-dawg-comments-request@w3.org>
> >>> To: public-rdf-dawg-comments@w3.org<public-rdf-dawg-comments@w3.org>
> >>> Sent: Sun May 01 20:29:37 2011
> >>> Subject: Proposal for simplifying FILTER semantics
> >>>
> >>> Dear WG,
> >>>
> >>> when working with SPARQL recently, I noticed that certain disjunctive
> >>> queries are most cumbersome/inefficient to formulate due to the special
> >>> post-processing semantics of FILTER expressions. I have written up a
> >>> detailed explanation [1]. In a nutshell: it is *really* hard to combine
> >>> FILTERs and BGPs in disjunctions.
> >>>
> >>> But the problem has a simple fix:
> >>>
> >>> * Define FILTER in such a way that it can *create* new solution
> >>> mappings, just like BGP. A FILTER would create all variable bindings (to
> >>> terms from the active graph) that make the filter condition true.
> >>> * Instead of applying filters after matching, the generated solution
> >>> mappings of a FILTER would directly be joined with other parts of the query.
> >>>
> >>> Putting it like this simplifies the whole algebra, both formally and
> >>> conceptually. Moreover, I think that practical implementation are
> >>> already working like that anyway (using FILTER conditions such as "=" to
> >>> pre-generate results instead of waiting until the very end before
> >>> "checking" them).
> >>>
> >>> The only negative effect that I see is that this would change the
> >>> meaning of variables that occur in filters but in no BGP. Currently,
> >>> such variables are considered "unbound". With the change, they would be
> >>> instantiated to all terms that match. Experimenting with FILTER-only
> >>> variables in some RDF stores, I merely got error messages (and rightly
> >>> so, since a variable that can never be bound is of little use in a
> >>> filter). So I assume that this is a corner case of little practical
> >>> relevance.
> >>>
> >>> AFAICT, all other queries would give exactly the same results (joining
> >>> having the same effect as filtering). So it seems that I am suggesting a
> >>> largely formal algebra change, but one that would make hitherto useless
> >>> queries very helpful (e.g. to solve the problem in [1]).
> >>>
> >>> I am aware that this proposal comes at a very late stage, but I think it
> >>> is still feasible to do it. I could help with updating the formal parts
> >>> of the algebra. In any case, I would like to hear the opinion of
> >>> implementers/practitioners, also re [1]. Note that I am writing this
> >>> largely as a user (and teacher) of SPARQL, so when I am investing my
> >>> time here it is merely because I am convinced that it would greatly
> >>> benefit the language.
> >>>
> >>> Cheers,
> >>>
> >>> Markus
> >>>
> >>>
> >>> [1] http://korrekt.org/page/The_State_of_the_UNION
> >>>
> >>
> >
> >
> 
>