# Re: Review SPARQL Query 1.1, Section 18 (algebra)

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Mon, 2 May 2011 23:17:22 +0100
Message-ID: <BANLkTinvL=OE-U7uUkZ6qiwq4GMj68MFEQ@mail.gmail.com>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>
```On 2 May 2011 21:10, Andy Seaborne <andy.seaborne@epimorphics.com> wrote:
[snip]
>
> 1/ http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#defn_algFilter
>
> Definition: Filter
> Filter(expr, Ω, D(G)) = { μ | μ in Ω and expr(μ) is an expression that has
> an effective boolean value of true }
>   Note that evaluating an exists(pattern) expression uses
>   the dataset and active graph, D(G).
>   See the evaluation of filter.

I now I am a pain in the arse, but I can't help it ;-)

We now have:
Definition: Filter
...
Filter(expr, Ω, D(G)) =

and:
Definition: Evaluation of Filter(F, P)
eval(D(G), Filter(F, P)) = Filter(F, D(G), eval(D(G),P))

In the Filter Def. D(G) occurs in the third position, whereas in the
Evaluation Def. it is used in the second position.

> 2/ http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#defn_evalFilter
>
> Definition: Evaluation of Filter(F, P)
>
> eval(D(G), Filter(F, P)) = Filter(F, eval(D(G),P), D(G))
>
> and moved the definitions for substitute and exists next to eval of Filter.
>
> I don't think it's helpful to rewrite the whole of expression evaluation to
> include D(G) but instead use language and highlight that the context of
> evaluation can provide it.

I am still not really happy with the way it is handled :-( Before
FILTER (NOT) EXISTS the algebra and its evaluation was quite
systematic; BGPs generate solutions and everything else was on top of
that, combining and working with sets of solutions. This nice
organisation is now gone :-( We have now a muddled interaction, which
relies on language to explain how all this is supposed to work.
Unfortunately, there's not much time to get anything better written up
and I guess neither you nor I (definitely not I) have the time to work
on this at the moment :-(

Birte
>
>        Andy
>
>

--
Dr. Birte Glimm, Room 309
Computing Laboratory