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Re: Review of Query document (basic federated query)

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Wed, 22 Sep 2010 15:31:34 +0100
Message-ID: <4C9A1346.8040401@epimorphics.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
>> > > 13 Basic Federated Query
>> > > See basic federated query doc
Much of the Federation document is written in a very casual and 
narrative fashion (very different than the Query doc; I suspect this 
will be very obvious if the federation text is just merged with the 
query document).

The document never discusses the "UNDEF" token that is introduced in the 
grammar.

"Solution Mapping (corresponds to the Concepts and Abstract Syntax term 
"RDF URI reference")" -- seems like a copy-paste typo.

"For instance, an edpoint" -- sp. "endpoint".

The examples are hard to follow because they are so domain-specific.

"The mechanics of executing a query over a graph" -- is this meant to be 
referring to "executing a query over a *named* graph"?

"Typically, a GRAPH constraint is matched against an RDF graph which is 
in the querying system, perhaps as the result of parsing the response to 
an HTTP GET on the named graph." -- This is needless detail. A GRAPH 
pattern is matched against named RDF graphs contained within the dataset 
being used for the query.

"GRAPH-constrained pattern" -- I don't know what this means.

"Note that WSDL defines the behavior with respect to constructing HTTP 
URLs from an endpoint and a set of query parameters, in particular 
appending '?' or '&' to an endpoint URL which may already have them." -- 
I'm not totally sure what this means, but I'd like to suggest that there 
should be a way to query over a custom dataset at the remote endpoint 
using the standard SPARQL Protocol conventions (SERVICE 
<http://example/endpoint?default-graph-uri=foo> {...}).

"application/sparql-results" -- should be "application/sparql-results+xml"

"For any other response, the query fails." -- Should this fail or just 
return an empty result set? I can think of arguments for both, but 
SERVICE blocks within OPTIONALS and UNIONS would be more useful if they 
didn't cause the entire query to fail.

"queryier" ??

In the example for section 3 BINDINGS, the ?id variable is bound to 
plain literals, but the example data from earlier in the document uses 
xsd:integer typed literals.

>> > > [FED]4.2 Definition of BINDINGS
"If a WhereClause has a BindingsClause" -- WhereClause doesn't 'have' a 
BindingsClause. The grammar associates these two through SelectQuery 
(with an intervening SolutionModifier).

Section 4.2 doesn't seem to follow the same conventions as the query doc 
. For example, "eval(BindingsSolutionSequence(P,V,St)) = Join(Rbc, P)" 
-- isn't P (a GGP) an AST, not an algebra, concept?.

>> > > [FED]5 SPARQL Federation Extensions Grammar
"""
It is a syntax error if to use a variable as the first argument to a 
ServiceGraphPattern if that variable is not bound (at least optionally) 
in the left hand side of a join with the ServiceGraphPattern on the right.
"""

"if to use" -- should be "to use"
This text should align with Axel's(?) proposed "potentially bound" 
concept, but in general it seems like it's trying to talk about a syntax 
error defined in terms of the algebra which is going to be confusing for 
people who otherwise don't need to ever think about the algebra. Also, 
join ordering doesn't have to use the lexical ordering, so "left hand 
side" here isn't particularly useful.
Received on Wednesday, 22 September 2010 15:10:24 GMT

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