Re: MIN and MAX in face of errors

Ah - I just sent a comment ….

SAMPLE is amusing … 

(SAMPLE(?x+0) AS ?y)

could be required to verify that ?x cannot be a nonnumeric or may be not …. I don't think the specification is clear.



Jeremy J Carroll
Principal Architect
Syapse, Inc.



On Oct 17, 2013, at 4:06 AM, Andy Seaborne <andy@apache.org> wrote:

> Recorded as
> 
> http://www.w3.org/2013/sparql-errata#errata-query-6
> 
> so it does not get lost.
> 
> (but this is does not stop discussion here).
> 
>  Andy
> 
> 
> On 15/10/13 19:00, Andy Seaborne wrote:
>> Hi Jeremy,
>> 
>> I was hoping someone would have replied to make their case because when
>> I looked at the details, I see different readings, including
>> counter-intuitive behaviour.
>> 
>> Intuitively, for a mix of numbers and errors, one might expect:
>> 
>>  -(MIN(-?x)) == MAX(?x)
>>  -(MAX(-?x)) == MIN(?x)
>> 
>>> I am not at all the sure that the spec is clear about these cases
>> 
>> Which sections of the spec in particular are unclear?
>> 
>> There's a not-endorsed-by-any-WG errata page at:
>> 
>> http://www.w3.org/2013/sparql-errata
>> 
>> as input to any future WG and corrections/improvements
>> 
>> - - - - - - - - - - -
>> 
>> Any errors in the BIND lead to the variable assigned to unbound, and in
>> particular, not an error value. 'error' as a new kind of value only
>> exists in aggregation.
>> 
>> 
>> MAX is more interesting that MIN here because MIN comes up with the same
>> answer for different readings; MAX does not.
>> 
>> Looking back over WG discussions, there was a lot of consideration of
>> what happens about mixed types, e.g. numbers and strings, leading to the
>> choice of the ordering relation for ORDER BY being used.  There is less
>> discussion about the impact of errors coming from group evaluation.
>> 
>> There are two problems I can see, both related to the introduction of
>> 'error' as an extension to the set of values (RDF terms) in aggregation.
>> 
>> 1/ ToList() is simply not defined for the case where there is an error.
>> 
>> You could argue that by SPARQL eval rules, the case of "not covered"
>> means it itself is an error so Min(ToList(Flatten(M)))
>> 
>> Or you could argument ToList is supposed to just create a list and pass
>> the error token through.  Then we have (2) below, with extended Ψ and
>> then MIN(Error, 1) is 1.
>> 
>> 2/ Similarly but less significantly because to matter it is a particular
>> reading of (1), the ordering relation for ORDER BY does not deal with
>> errors as input; it takes expressions and a solutions, evaluates the
>> expressions and that may be an error.  OrderBy(Ψ, condition) and Ψ can't
>> contain error. (sec 18.5).
>> Only the case of "condition(μ)" being an error is covered.
>> 
>> ---------------------------
>> 
>> COUNT specifically calls errors out
>> 
>> N = Flatten(M)
>> remove error elements from N
>> 
>> SUM, AVG -- these end up with error either way round if we assume
>> op:numeric-add(error, ...) is defined analogously to ToList(error)
>> needed for that reading.
>> 
>> GroupConcat calls CONCAT so same here even though it does not use ToList.
>> 
>> SAMPLE does not use ToList; it contains the line:
>> 
>> """
>> the only restriction is that the output value must be present in the
>> input multiset.
>> """
>> 
>> Can SAMPLE({"a", "b", "c", error}) validly return error?
>> 
>> ---------------------------
>> 
>> This is worth an entry in the errata collection.
>> 
>>     Andy
>> 
>> On 10/10/13 18:21, Jeremy J Carroll wrote:
>>> 
>>> 
>>> I am wondering what is the right answer to the following query
>>> 
>>> SELECT
>>>   (MIN(?x) as ?a)
>>>   (MAX(?x) as ?b)
>>>    (MIN(?x+0) ?c)
>>>    (MAX(?x+0) AS ?d)
>>> {
>>>   { BIND(1 as ?x)}
>>>   UNION
>>>   { BIND(1 +"x" as ?x)}
>>>   UNION
>>>   { BIND("y" as ?x)}
>>> }
>>> 
>>> Note that the middle BIND creates an unbound ?x, (which is lower
>>> than1 or "y"), whereas the last BIND creates an error inside the 2nd MIN
>>> and MAXes … (when ?x="y")
>>> 
>>> I am not at all the sure that the spec is clear about these cases
>>> 
>>> Jeremy
>>> 
>>> 
>>> Principal Architect
>>> Syapse, Inc.
>> 
> 
>

Received on Thursday, 17 October 2013 17:07:59 UTC