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Re: Comments on SPARQL (based on an SPARQL Engine implementation in Python)

From: Ivan Herman <ivan@w3.org>
Date: Sat, 06 Nov 2004 14:24:37 +0100
Message-ID: <418CD095.5090203@w3.org>
To: "Seaborne, Andy" <andy.seaborne@hp.com>
Cc: public-rdf-dawg-comments@w3.org, Daniel Krech <eikeon@eikeon.com>
Andy,

thanks for the reply. Some (small) answer/remarks below

Seaborne, Andy wrote:
> 
>>
>>--------------
>>
>>Constraining Values.
>>
>>The draft refers to the possibility of using application
>>methods/functions as constraints. And that is good. However, the
>>question arises: what bound variables the function has access to? In
> 
> my
> 
>>implementation I separated a 'per-pattern' constraint and a 'global'
>>constraint. Per pattern constraint functions are invoked for one
>>triplets, and get the three (bound) triplet resource references as
>>arguments. 'Global' constraint functions are invoked at the end of the
>>full pattern matching process, and they have access to all the binding
>>(eg, in the case of Python, in the form of a dictionary of the form
>>{"?x" : TheBoundResourceForX,... }.  
>>
>>Clearly, the global constraint can, functionally, replace the
>>per-pattern constraint, but using per-pattern constraint may make the
>>implementation way more efficient (essentially, it may cut what I
>>called the expansion tree early on in the process). It is also clear
>>that a query language parser can separate per-pattern constraints for
>>the kind of examples that are in the draft (?a < 10, etc), so it may
> 
> be
> 
>>enough if the underlying engine offers this differentiation. But a
>>parser cannot cover the general user method case. The issue is whether
>>we want to make that differentiation or not in SPARQL or not for this
>>reason. In any case, this question *must be specified* in the
> 
> document,
> 
>>imho.   
> 
> 
> This is an area where the working draft says very little and it is being
> worked on at the moment.
> 
> Constraints, like triple patterns, are restrictions on the results that
> match the query.  For any match, there is a set of bindings and
> constraints evaluated on these function must be true.
> 
> A function will receive its variables as arguments (and the variables
> may be unbound due to optional, say - the function will have to cope).
> The actual mechanism for doing that will be implementation dependent -
> executing all the constraints after the triple pattern matching would be
> correct but may be slower than executing a constraints as soon as its
> variables can be bound, e.g., just after the point where all varibales
> used have been mentioned in patterns as per your per-pattern
> constraints.  As a query engine may choose to match the triple patterns
> in any order, or in groups, there is lots of scope for implementation
> optimization here.
> 

Understood. Actually, this is in line with what I implemented: on the 'engine' level there 
is a possibility to call a constraint method per triplets as well as for global, and an 
SQRL parser may optimize by choosing the former when appropriate


> 
>>---------------
>>
>>Nested Patterns. It is not clear in the draft how 'deep' nesting can
>>go. I did only the simple one, ie, only a one level depth is managed:
>>
>>(?a,?b,c), {(?q,?w?,?r),(?s,t,?u)}
>>
>>It is not clear whether a nesting of the kind:
>>
>>(?a,?b,c), {(?q,?w?,?r),{(?s,t,?u),(?q,k,?o)}}
>>
>>is also allowed or not. Actually, if it is, it has to be defined what
>>it really *means*.
> 
> 
> In that example, it is all conjunction and the same as:
> 
> (?a,?b,c) (?q,?w?,?r) (?s,t,?u) (?q,k,?o)
> 
> 


Then I am lost.:-( My understanding was that (just referring to Ti-s for triples):

[T1 {T2 T3}] means [T1 T2] OR [T1 T2]

so why would

[T1 {T2 {T3 T4}}] mean [T1 T2 T3 T4] as you write? What is then the difference between

[T1 {T2 {T3 T4}}] and [T1 {T2 T3 T4}] or indeed [T1 T2 T3 T4]?

My interpretation of [T1 {T2 {T3 T4}}] would be


[T1 T2] OR [T1 T3 T4]

what am I missing?

> 
>>(My initial thoughts are that it means an
>>alternation of 'or'-s and 'and'-s, it means 
>>
>>(?a,?b,c) and (?q,?w?,?r)
>>or
>>(?a,?b,c) and (?s,t,?u) and (?q,k,?o)
>>
>>etc, recursively.) If this is adopted, it has to be described.
> 
> 
> Noted.
> 
> 
>>-----------------
>>
>>Optional Patterns. I was not clear to me *why* there might be more
> 
> than
> 
>>one optional patterns, whereas there is only *one* where pattern. Why
>>the asymmetry? 
> 
> 
> There may be more than one optional because there may be indendent
> optionalk information:
> 
> PREFIX foaf: <http://xmlns.com/foaf/0.1/>
> SELECT ?name ?mbox ?hpage
> WHERE  ( ?x foaf:name  ?name )
>        [ ( ?x foaf:mbox ?mbox ) ]
>        [ ( ?x foaf:homepage ?hpage ) ]
> 
> Here, "mbox" and "hpage" get added to the results independently of each
> other.
> 

I presume you meant
 > PREFIX foaf: <http://xmlns.com/foaf/0.1/>
 > SELECT ?name ?mbox ?hpage
 > WHERE  ( ?x foaf:name  ?name )
 > OPTIONAL  [ ( ?x foaf:mbox ?mbox ) ]
 >           [ ( ?x foaf:homepage ?hpage ) ]

but is the difference between that and

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?mbox ?hpage
WHERE  ( ?x foaf:name  ?name )
OPTIONAL  { ( ?x foaf:mbox ?mbox ) ( ?x foaf:homepage ?hpage ) }


Ie, doesn't the nesting possibility already give you what you are looking for?


> 
>>I actually wonder whether it is not better to define the combination
> 
> of
> 
>>query results in general (one could imagine the sum of two queries,
>>being the concatenation of the result lists) and let the individual
>>queries having a simpler structure instead. Just a thought. 
>>
>>-----------------
>>
>>Query patterns. Not surprisingly, this part is a bit vague (and that
> 
> is
> 
>>*no* critique on the editors, it is just the natural status of
> 
> things).
> 
>>My understanding of the CONSTRUCT * is based on an old note of Guha &
>>al[3] (thanks to DanBri who drew my attention on it). Is this the
> 
> right
> 
>>interpretation? 
> 
> 
> It is like that but not exactly - it wouldn't add extra information as
> does one of the examples in the paper you reference.  It is the merge of
> query pattern with the variables subituted into the pattern.  DESCRIBE
> is there to allow extra server-decided information to be returned.
> 

Right. My reference was actually wrong, my understanding is exactly what you describe. Thanks.


> 
>>I found it a bit difficult to mentally bind the CONSTRUCT stuff with
>>the rest of the document. It stands a bit separate from the rest. My
>>abstraction in Python was, instead, that a query (select, where,
>>optional, etc), returns in fact a query result object, and then
> 
> select,
> 
>>construct, etc, are just methods on that Object. I wonder whether a
>>similar notion may not work better when describing the intentions. 
>>
>>-----------------
>>
>>Finally, the missing bits. Both in SPARQL[1] and in the requirement
>>document[4] I was desperately looking for a way to manage collections
>>and containers. SPARQL does *not* give me a way to ask whether '?x' is
>>part of the collection 'C', or of the Seq 'S'. For handling any of
>>these cases one has either to introduce some form of non-finite query
>>or make some special forms for these, specifically. But none of this
> 
> is
> 
>>documented. For example, if I want to use SPARQL to query into RDF
>>graph of a specific OWL ontology, and I want to find out whether a
>>specific class 'C' is part of the 'unionOf' describing the class 'D',
> 
> I
> 
>>hit this problem....  
> 
> 
> This is on the issues list (issue "accessingCollections").
> 
> 

Oops, I missed that one. Apologies.



>>-----------------
>>
>>I hope these remarks are helpful
> 
> 
> Yes thanks,
> 
> 	Andy
> 
> 
>>Ivan
>>
>>P.S. Disclaimer: though I am part of the W3C Team, this SPARQL
>>implementation has not been done as part of a 'formal' W3C project,
> 
> ie,
> 
>>it does not reflect some sort of a W3C opinion! Rather, it is a
>>spin-off of some other things I did using RDF. 
>>
>>[1]http://www.w3.org/TR/2004/WD-rdf-sparql-query-20041012/
>>[2]http://www.ivan-herman.net/Python/sparqlDesc.html
>>[3]http://www.w3.org/TandS/QL/QL98/pp/enabling.html
>>[4]http://www.w3.org/TR/2004/WD-rdf-dawg-uc-20041012/
>>
>>
>>
>>
>>--
>>
>>Ivan Herman
>>W3C Communications Team, Head of Offices
>>C/o W3C Benelux Office at CWI, Kruislaan 413
>>1098SJ Amsterdam, The Netherlands
>>tel: +31-20-5924163; mobile: +31-641044153;
>>URL: http://www.w3.org/People/all?pictures=yes#ivan
> 
> 
> 

-- 

Ivan Herman
W3C Communications Team, Head of Offices
C/o W3C Benelux Office at CWI, Kruislaan 413
1098SJ Amsterdam, The Netherlands
tel: +31-20-5924163; mobile: +31-641044153;
URL: http://www.w3.org/People/all?pictures=yes#ivan

Received on Saturday, 6 November 2004 13:24:39 GMT

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