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Re: Filter scope and optionals

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Tue, 31 Oct 2006 11:14:33 +0000
Message-ID: <45473019.9090603@hp.com>
To: jperez@ing.puc.cl, public-rdf-dawg-comments@w3.org



Jorge Pérez wrote:
> Hi Andy,
> 
>> If the proposed filter scoping rule is taken naively, there isn't a way to
>> get
>> value testing conditions into the left join conditions for optional
>> (unlike
>> the join conditions for a group).
>>
>> Here's an example:
>>
>> ---- Data
>> @prefix   :         <http://example/> .
>>
>> :x1 :p 1 .
>> :x2 :p 2 .
>>
>> :x3 :q 3 .
>> :x3 :q 4 .
>> ---- Data
>>
>>
>> ---- Query
>> PREFIX :    <http://example/>
>>
>> SELECT *
>> {
>>    ?x :p ?v .
>>    OPTIONAL
>>    {
>>      ?y :q ?w .
>>      FILTER(?v=2)
>>    }
>> }
>> ---- Query
> 
> I personally think that this kind of queries are interesting from the user
> point of view, but because they need a *dynamic scope* evaluation (I
> really don't know how to call it, but I think you understand) the language
> looses the compositionality. What I mean is that the *expression*
> 
> {
>   ?y :q ?w .
>   FILTER(?v=2)
> }
> 
> has different meanings depending on where it appears on the query. Here
> you are discussing the appearance of it in the right hand side of an
> OPTIONAL, but it would appear in a parallel group, in a UNION, inside a
> GRAPH, etc. Also here the expression in the filter is very simple but I
> think that with bound\!bound things could be more complex.
> 
>>
>> ---- Results 1
>> ---------------------
>> | x   | v | y   | w |
>> =====================
>> | :x2 | 2 | :x3 | 4 |
>> | :x2 | 2 | :x3 | 3 |
>> | :x1 | 1 |     |   |
>> ---------------------
>>
>> ---- Results 2
>> -------------------
>> | x   | v | y | w |
>> ===================
>> | :x2 | 2 |   |   |
>> | :x1 | 1 |   |   |
>> -------------------
>>
>> Results 1 is what the current doc (declarative form) gets - and would be
>> the
>> same if the FILTER were part of the left outer join conditions.
>>
>> Results 2 is what a purely filter-scoped-to-group gets because the
>> optional
>> part never matches because ?v is an unbound variable.
>>
>>
>> I can't see any way of getting filter into the scope of the left outer
>> join.
>> The only way I can see of rewriting this query is to use a UNION and
>> repeat
>> the fixed pattern on each side of the union, one side including the filter
>> and
>>   optional triple pattern. That query is going to give different answers
>> and
>> the app will have to post-process the results to find out what went on.
> 
> And the following query?
> 
> SELECT *
> {
>    ?x :p ?v .
>    OPTIONAL
>    {
>      ?x :p ?v .
>      ?y :q ?w .
>      FILTER(?v=2)
>    }
> }
> 
> I think (but I do not have a proof of it), that giving a dinamic scope for
> filters in optionals does not increase the expressiveness of the language
> compared with the compositional-algebraic-operations approach, and one can
> always obtain the same result repeating the pattern inside the optional
> (as in the above query).

It may not change the expressivity but it isn't convenient for the application 
writer.  The example here has one triple pattern - in a realistic query, it 
would be 10's of triple patterns.  Repeating it would be a significant burden 
and potentially error prone, for example if the repeated pattern needs changing.

It does suggest an approach though, which is to define optional, not directly 
as a left join operation but to do that repetition:

OPT(A,B) = LeftJoin(A, Join(A, B))

but filters need to be handled so that itself does not work.

I haven't worked though properly but an idea:

Write Group(P,F) where P a pattern, F is a filter restriction,
scope filters over the evaluation of the pattern:
then Group(P,F) = F(P) = [[ [[P]] FILTER F ]]

so:

OPT(Group(P1, F1), Group(P2,F2)) =
     LeftJoin(F1(P1), F2( Join(F1(P1), P2) ))

and F2 can mention variables in P1

which is  F1(P1) LeftJoin (F1(P1) Join P2) ON (F2 and matching vars)
(in a vaguely SQL-ish notation)

which looks like it might be  F1(P1) LeftJoin P2 ON (F2 and matching vars)

If F2 only mentions variables from P2 and not P1, then it can be moved in to 
apply directly to P2.

If F2 mentions a variable x from P1 and P2, then x must have the same value in 
each so its the same as applying F2 to P2 alone.

(Caveat - design-by-email.  Further work and testing needed!)


> 
>> Intuitively, the scope rule needs to be modified to put the filter just
>> outside the group, allowing it to be part of the left outer join, and not
>> just
>> inside the group.
> 
> I think it is not so simple, because you can have patterns like
> 
> { ?x :p ?v
>   OPTIONAL {
>      ?y :p ?w
>      OPTIONAL {
>         ?z :q ?u .
>         FILTER (?v=2) } } }
> 
> and then the problem affect the semantics of nested optionals too.

I wasn't thinking of extending the scoping to allow this.  I don't see it 
happening for real nearly as much as the one level form above.

By allowing the scope to be just outside the containing optional part, the 
FILTER in the innermost optional does not have access to the outermost "?x :p 
?v" and ?v is unbound for the restriction.


> 
>> Any alternatives?
>> How do we formalize this?
> 
> One approach is to formalize OPTIONAL + FILTER in a navigational way (as
> the current spec).
> Another approach to formalize the dinamic scoping (without loosing
> compositionality) is to think that the filter is *generating bindings*,
> for example from
> 
> {
>   ?y :q ?w .
>   FILTER(?v=2)
> }
> 
> results the solutions for ?y :q ?w defined also in variable ?v but with
> fixed value 2 for ?v. What I do not like of this approach is that FILTER
> is not filtering, it is *forcing* some bindings, and formal definitions
> would become complex in the case of restrictions like ?x = ?y, or
> restrictions with negations.

I don't like this either (FILTER(?v < 3) for example) but it does show the 
asymmetry here: variables mentioned in the pattern play a part in the 
left-join condition but variables mentioned in the filter don't.

> 
> I'm personally in favor of the Results 2 above (filter-scoped-to-group).
> 
> - jorge
> 
> 
>> 	Andy

Thanks for the prompt comments

	Andy

>>
>>
> 
> 
Received on Tuesday, 31 October 2006 11:44:45 GMT

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