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Re: Review of Query document (Property paths, negation, select expression, testing values)

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Wed, 22 Sep 2010 15:05:42 +0100
Message-ID: <4C9A0D36.6020708@epimorphics.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>

On 20/09/10 23:18, Gregory Williams wrote:
> I've based the review on the sections Andy highlighted in a previous email as having been changed. His sections are quoted below (with some additional section headers added for the federation document).
> .greg

Greg - thank you very much for the review and comments.

To make it easier to track all the comments across different areas of 
prime responsibility, I've split the content up into:

* Property paths, negation, select expression, testing values
* Groups/aggregates & subquery
* Basic federated query.


 >> > > Material different from SPARQL 1.0:
 >> > >
 >> > > 8 Negation
 >> > >    8.1 Filtering Using Graph Patterns
 >> > >        8.1.1 Testing For the Absence of a Pattern
 >> > >        8.1.2 Testing For the Presence of a Pattern
 >> > >    8.2 Removing bindings
 >> > >    8.3 Relationship and difference between NOT EXISTS and MINUS
I find the naming of 8.1 and 8.2 to be asymmetrical. 8.2 seems like it's 
more about removing query solutions, not variable bindings. Section 8.2 
also says, "calculates solutions in one side that are not compatible 
with the other side" but this isn't exactly true. The text needs to 
explicitly call out that it's not just compatibility but also a 
non-empty domain intersection that is required for the result removal. 
The specifics of this are in 17.4. However, I think the wording in 17.4 
is misleading:

The additional restriction on dom(μ) and dom(μ') is added so that if any 
solution mapping has no variables in common with solution mappings of Ω1 
then Minus(Ω1, Ω2) is empty, regardless of the rest of Ω2.

But "then Minus(Ω1, Ω2) is empty" should really be "then Minus(Ω1, Ω2) 
is NOT empty," distinguishing it from the case that would result in 
Minus just determining compatability without concern for the domains of 
Ω1 and Ω2. Also, the cardinality definition for Minus seems to say that 
it's the same as the cardinality of the Minus operation's left-hand 
side, but that doesn't seem right.

 >> > > 9 Property Paths
The first sentence of section 9 seems to be missing its ending period.

Outside of the grammar rules interspersed throught the document, the 
word "property" is never used in relation to the predicate of a 
triple(pattern) before the property paths section. This may make the 
text confusing in discussing property paths as "similar to a string 
regular expression but over properties, not characters". Maybe the 
relation between properties and predicates is obvious enough to not need 
clarification, but I thought it worth noting.

There's a free-floating sentence fragment just before section 9.1: "any 
given path expression."

 >> > >    9.1 Property Path Expressions
"uri is either a URI or a prefixed name" -- is the 'a' shorthand for 
rdf:type allowed?

I think "nor uri_{j+1}...^uri_n as reverse paths" is missing a ^ on 

 >> > >    9.2 Examples
"or, with explicit variables" might be more clear with text indicating 
that the extra variables make this not exactly the same query as the 
results from this variant will include bindings for the new named variables.

"rdf;type" should be "rdf:type"

 >> > > These next sections contain material that will be moved into the 
formal definitions:
 >> > >
 >> > >    9.3 Algebra for Property Paths
 >> > >        9.3.1 Translation of Property Paths
 >> > >        9.3.2 Material for the "Initial Definitions" section
 >> > >   Property Paths Patterns
"A Property Path is a sequence of triples, ti in ST" -- what is "ST"?

"We call the object of t_n the end of the path." -- should n here 
instead be length(ST)-1?

 >> > >        9.3.3 Property Path Expression Matching
 >> > >   SPARQL Property Path Pattern Matching
 >> > >
 >> > >        9.3.4 Material for the SPARQL syntax to SPARQL algebra 
 >> > >        9.3.5 Material for the SPARQL algebra section
"A zero length path matches subjects and all objects ..." Does this mean 
"all subjects and objects"? Not sure why the choice of "subjects" vs. 
"*all* objects".

"such that a intermediate nodes in the graph are trarversed once only" 
-- 'a' should be 'all'?

 >> > >        9.3.6 Material for the SPARQL Evaluation Semantics section
 >> > >   Zero-length paths
Is there a reason to include 'path' in ZeroLengthPath(X, path, Y) ?

"{ { (y, x) } }" -- should be "{ { (vy, x) } }"

eval(D(G), ZeroLengthPath(x:term, path, y:term)) =
   { {} }
   card[] = 1
-- should this return the empty set {} instead of the set of one empty 
result { {} } when x != y?

 >> > >   Arbitrary length paths
"deteching" -- sp. "detecting".

"ALP(x:term, path) = ALP(x:term, path), {})" -- has one too many 
right-parens in it. Presumably should be "ALP(x:term, path) = 
ALP(x:term, path, {})"

"R = R + t + ALP(t, Path, Visted)" -- should 't' be '{t}'? (assuming + 
is understood as set-union?)

"multiset-union({(vx,t} | | t in nodes(G) and y in 
ArbitraryLengthPath(t, path, y) })" -- syntax here is messed up, and I'm 
not sure about the second (recursive?) condition 'y in 
ArbitraryLengthPath(t,path,y). Maybe meant to be "t in nodes(G) and y in 
ALP(t,path)" ?

 >> > >        15.1.2 SELECT expressions
"Variables can be also be used in expressions if they are introduced as 
to the earlier, syntactically, in the same SELECT clause" -- the wording 
here needs to be fixed. Talking about "New variables" being available 
might be more clear than just "Variables".

 >> > >    16.6 Extensible Value Testing
 >> > >
 >> > > New functions where there is a keyword:
 >> > >        16.4.14 COALESCE
 >> > >        16.4.15 IF
"rdfTerm coalesce(expression*)" -- this syntax doesn't seem to include 
the (required?) commas to separate expressions (c.f. 16.4.16 IN).

"The COALESCE function form" -- why not just "COALESCE function" without 
"form"? Similarly for IF.

"COALESCE(5, ?x)	returns 2" -- shouldn't it return 5?

"the returns the value of expr2" -- should be "*then* returns the value 
of expr2".

 >> > >        16.4.16 IN
"A list of zero terms on the right-hand side is legal." -- this is 
repeated twice in this section.

 >> > >        16.4.17 NOT IN
"A list of zero terms on the right-hand side is legal." -- this is 
repeated twice in this section.

 >> > >        16.4.18 IRI
"Passing any other RDF term is an error." -- this is misleading as both 
simple literals and iris are allowable, but this directly follows a 
sentence describing just the IRI(iri) case.

 >> > >        16.4.19 URI
 >> > >        16.4.20 BNODE
"constructs a blank node that is distinct for all blank nodes" -- should 
that be "distinct *from* all blank nodes"?

"distinct from all blank nodes created by calls to this constructor for 
other query solutions." -- "other query solutions" seems strange here 
since at the time BNODE() is evaluated it might not be operating with an 
actual query solution, but just a set of variable bindings (if invoked 
in a FILTER in the middle of a query). Is "query solution" still a valid 
term for these intermediate 'solutions'?

"and the same blank node for calls with the same simple literal within a 
FILTER for one solution mapping" -- is there a way to get the same bnode 
for the same simple literal *across* different solution mappings (across 
the entire query execution)?

 >> > >        16.4.21 STRDT
 >> > >        16.4.22 STRLANG
STRLANG("chat", "en")	"123"@en

-- should be "chat"@en, not "123"@en.

 >> > >        16.4.23 NOT EXISTS and EXISTS
"returns true/false depending on whether the pattern matches" -- Matches 
what? More detail would help explaining that it returns true if the 
evaluating the pattern (with variable substitution) results in any 

"The NOT EXISTS form translates into fn:not(EXISTS{...})." -- is there a 
reason to prefer this to "!(EXISTS{...})"?

"Returns false if pattern pat matches the dataset."
"Returns true if pattern pat matches the dataset."
"Variables in the pattern pat"
-- all of these refer to 'pat', but the texts they are referring to use 

"Let μ a solution mapping" -- should be "Let μ *be* a solution mapping"
Received on Wednesday, 22 September 2010 15:11:30 UTC

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