- From: Axel Polleres <axel.polleres@deri.org>
- Date: Fri, 26 Mar 2010 09:57:16 +0000
- To: Andy Seaborne <andy.seaborne@talis.com>
- Cc: SPARQL Working Group <public-rdf-dawg@w3.org>

short clarification request: > { eval(expr,μ) | μ in Ω such that eval(μ(expr)) is defined } by "is defined" you mean "is unequal to 'error'", yes? What I mean to ask here... when I read the current section on Filter evaluation http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#evaluation to me it seems that eval is always "defined", but it could be an error for reasons of mistyping or values being unbound. Thanks for clarification on whether/what I might have overlooked here! Axel On 7 Mar 2010, at 17:42, Andy Seaborne wrote: > ISSUE-53 > > I propose the following to define ExprMultiSet: > > ------- > > Let Ω be a partition. > > ExprMultiSet(Ω) = > { eval(expr,μ) | μ in Ω such that eval(μ(expr)) is defined } > UNION > { e | μ in Ω such that eval(μ(expr)) is undefined } > > where "e" is some symbol that is distinct from all RDF terms. > > card[x]: > if DISTINCT: > card[x] = 1 if there exists μ in Ω such that x = eval(μ(expr)) > card[x] = 0 otherwise > else > card[x] = count of μ in Ω such that x = eval(μ(expr)) > > -------- > > "e" just records error evaluations. > > This is the most flexible definition. An alternative is > > ExprMultiset(Ω) = > { eval(expr,μ) | μ in Ω such that eval(expr,μ) is defined } > > which is hard-coding dropping errors and unbounds during evaluation. But > the aggregate can't know there were some errors. > > Another possibility is that a yes/no flag indicating a error was seen. > But this might as well be the count of errors, which is equivalent to > the flexible definition given. > > By the way, this is in no way a recipe for implementation. Aggregation > can be done over all groups in parallel during query execution. > > > > For the last publication, it was noted > > http://lists.w3.org/Archives/Public/public-rdf-dawg/2009OctDec/0646.html > > Unbound and error are the same. The current design so far has it that > any error means that the multiset is invalid and that group is not > considered. > > We didn't have time to propose a solid design to address ISSUE-53 - the > potential design at the time of publication was that any error when > calculating the ExprMultiset from a partition meant that > > SUM of {1, 2, unbound} is an error. > COUNT of {1, 2, unbound} is an error. > > I don't think that is a useful form for COUNT(?x). It does seem to mean > that COUNT(?x) is either COUNT(*) or error; it can't be anything else. > > COUNT(?x) can not be zero because zero arises when there are no ?x but > there are solutions in the partition. If there are no solutions in the > partition then there is no group key and no grouping happens. > > For each aggregate we can decide what happens about unbounds and errors. > > I would like to see: > > COUNT(*) = size of multiset. > COUNT(DISTINCT *) = size of set after removing any e (i.e. skip undefs). > > COUNT(?x) = number of times ?x is defined in each group > 0 <= COUNT(?x) <= COUNT(*) > > COUNT(DISTINCT ?x) = number of times ?x is uniquely defined in each group > > I'm less worried about SUM(?x) but I'd prefer that > > SUM(?x) = op:numeric-add of defined values of ?x, skips unbounds > > rather that the rigid form we currently have. > > Previously, one of the difficulties raised for this design was that the > operation to add two numbers wasn't op:numeric-add because that could > not cope the errors (there were related datatyping issues as well). > > With the definition of ExprMultiSet above, op:numeric-add can be used to > define SUM. There is step between getting the ExprMultiSet and the > calculation of aggregation. This step, for SUM (and COUNT(?x)), removes > any errors. > > GROUP_CONCAT(?x) = concatenation > and now GROUP_CONCAT of an empty set can be defined as "". > > ------------- > Some examples: > > Does anyone want to suggest we design to get different results in any of > these cases? > > > --Data: > > @prefix : <http://example/> . > > :x1 a :T . > :x1 :p 1 . > :x1 :p 2 . > > :x2 a :T . > :x2 :p 9 . > > :x3 a :T . > :x3 :p 5 . > :x3 :q "x" . > > :x4 a :T . > :x4 :q "z". > > > -- > > > -- Query 1: > 1 PREFIX : <http://example/> > 2 > 3 SELECT ?x (count(*) AS ?C) > 4 WHERE > 5 { ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> :T > 6 OPTIONAL > 7 { ?x :p ?v} > 8 } > 9 GROUP BY ?x > 10 ORDER BY str(?x) > > ----------- > | x | C | > =========== > | :x1 | 2 | > | :x2 | 1 | > | :x3 | 1 | > | :x4 | 1 | > ----------- > > -- Query 2: > > Change line 3 to: > SELECT ?x (count(?v) AS ?C) > > ----------- > | x | C | > =========== > | :x1 | 2 | > | :x2 | 1 | > | :x3 | 1 | > | :x4 | 0 | > ----------- > > -- Query 3: > > Change line 3 to: > SELECT ?x (sum(?v) AS ?C) > > ----------- > | x | C | > =========== > | :x1 | 3 | > | :x2 | 9 | > | :x3 | 5 | > | :x4 | 0 | > ----------- > > The :x4 row is zero because there were no valid numbers to add together. > > -- Different query OPTIONAL part - now has ?p > > 1 PREFIX : <http://example/> > 2 > 3 SELECT ?x (sum(?v) AS ?C) > 4 WHERE > 5 { ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> :T > 6 OPTIONAL > 7 { ?x ?any ?v} > 8 } > 9 GROUP BY ?x > 10 ORDER BY str(?x) > > ----------- > | x | C | > =========== > | :x1 | 3 | > | :x2 | 9 | > | :x3 | 5 | > | :x4 | 0 | > ----------- > > The case where ?v is "Z2 and "x" have been skipped. > > Andy > > > > > >

Received on Friday, 26 March 2010 09:57:52 UTC