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Re: FW: Grammar updated to reflect BGP/Constraint interpretation

From: tessaris <tessaris@inf.unibz.it>
Date: Tue, 24 Jan 2006 15:02:39 +0100
Message-ID: <43D6337F.9040904@inf.unibz.it>
To: "Seaborne, Andy" <andy.seaborne@hp.com>
CC: RDF Data Access Working Group <public-rdf-dawg@w3.org>


Seaborne, Andy wrote:
> 
> Should have been send to the WG list.
> 
> 	Andy
> 
> -------- Original Message --------
> 
>>From: Seaborne, Andy <>
>>Date: 23 January 2006 13:27
>>
>>I have chnaged the grammar to reflect the clarification of BGPs and
>>constraints by putting a rule in that is called BasicGraphPattern. 
>>
>>The case of:
>>{ _:a :p ?v . FILTER(?v <3) . _:a : ?w }
>>
>>I have changed:
>>  Moved constraint into a BasicGraphPattern rule
>>  Created BlockOfTriples for a sequnece of triple patterns (this was
>>"Triples")
>>  Moved BlockOfTriples and BasicGraphPattern to be with the other
>>  pattern rules Renamed Triples1 to TriplesSameSubject
>>  Removed recursion in TriplesSameSubject
>>
>>	Andy

Andy, I understand the reasons for the changes but I'm not sure whether
they match everybody's intuition.

I think that we agree on the fact that SPARQL operators transform answer
sets (set of pattern solutions) rather than single pattern solutions.

Under this assumption I see FILTER as the relational (not SQL) SELECT
operator. So I don't see any reason for restricting its usage to Basic
Graph Pattern only. E.g. I think that a query like

   { BGP1 UNION BGP2 } FILTER (E1)

should be fine; but it seems to me that the new grammar is preventing this.

When during the telecon I said that 'FILTERS' can be pushed at the end I
meant that

{ _:a :p ?v . FILTER(?v <3) . _:a : ?w }

should be understood as

{ _:a :p ?v . _:a : ?w } FILTER(?v <3)

I'm not sure how to reflect this on the actual grammar, though.

Then I think that having the definition

""""
Definition:  Basic Graph Pattern

A Basic Graph Pattern is a set of Triple Patterns.
"""

together with the grammar rule

[21] BasicGraphPattern ::= BlockOfTriples? ( Constraint '.'?
BasicGraphPattern )?

looks confusing; since we have Constraints as well as triple patterns.

I'd rather prefer something closer to the old grammar, to be sure that a
Basic Graph Pattern is always a leaf in the parse tree:

[20] GraphPattern ::= FilteredBasicGraphPattern? (
GraphPatternNotTriples '.'? GraphPattern )?
[21] GraphPatternNotTriples ::=  OptionalGraphPattern |
GroupOrUnionGraphPattern | GraphGraphPattern | Constraint

with the rule

FilteredBasicGraphPattern ::= BlockOfTriples? ( Constraint '.'?
FilteredBasicGraphPattern  )?

and a comment on the fact that 'BoT1 constraint BoT2' should be
considered equivalent to '{ BoT1 . BoT2 } constraint'.


Cheers,
--sergio
Received on Tuesday, 24 January 2006 14:02:58 GMT

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