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Re: SUM aggregate operator and non-numeric literals

From: Steve Harris <steve.harris@garlik.com>
Date: Mon, 27 Jun 2011 13:20:20 +0100
Cc: public-rdf-dawg@w3.org
Message-Id: <3375BFD7-3A1D-4274-8035-4F52CCEC436A@garlik.com>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
On 2011-06-27, at 12:38, Andy Seaborne wrote:

> Looks fine - good point that SUM can be extended rather than requiring it's formulation to be changed.
> 
> 
> One observation (does not the reply being sent):
> 
> [[
> > The result will be always a type error.
> 
> Correct.
> ]]
> 
> I'm not sure what Jeen means here - the query does not produce a type error.  One of the two groups works, the other leads to an error in SUM but which caught in the SELECT expression and leads to an unbound variable.

I thought he was referring to the result of the Sum() set function, but I can see it's open to interpretation.

- Steve

> On 27/06/11 11:36, Steve Harris wrote:
>> I've added http://www.w3.org/2009/sparql/wiki/CommentResponse:JB-6
>> 
>> - Steve
>> 
>> On 2011-06-27, at 11:02, Steve Harris wrote:
>> 
>>> On 2011-06-26, at 16:43, Andy Seaborne wrote:
>>> 
>>>> (can we have one x.y.z. section per aggregate like the functions so each has it's own doc link or links to the defintion of each?  The defs don't have anchors).
>>> 
>>> Yes, good idea, I've added it to my todo list.
>>> 
>>>> There are 3 points where customization can be done:
>>>> 1/ The expression to be SUM'ed
>>>> 2/ The aggregate called
>>>> 3/ Whether SUM uses + or op:numeric-add
>>>> 
>>>> 1 and 2 can already be done.
>>>> 
>>>> 
>>>> For 1/:
>>>> 
>>>> The effect Jeen uses as an example can be achieved by ensuring the multiset passed to SUM are all numbers:
>>>> 
>>>> SUM(xsd:integer(?val))
>>> 
>>> Right.
>>> 
>>>> assuming that
>>>> 
>>>> :a rdf:value "rst" is still to be an error for the group it's in.
>>>> 
>>>> SUM(COALESCE(xsd:integer(?val),0))
>>>> 
>>>> would give the effect of skip any error.
>>>> 
>>>> Now, if the application does want an error, it is able to choose the expression.
>>>> 
>>>> Another way, which copes with a wider range of numbers but without over-promotion of types:
>>>> 
>>>> SUM(IF(isNumeric(?val), ?val , COALESCE(xsd:double(?val),0))
>>>> 
>>>> For 2/:
>>>> 
>>>> We do allow extension of aggregates via custom aggregates called by URI.  A different aggregation function operation would call a different operation.
>>>> 
>>>> For 3/:
>>>> 
>>>> 3/ A change to SUM so it uses "+"
>>>> 
>>>> Doesn't that constrain how "+" can be extended?  I presume it must be commutative and associative so order of the SUM aggregation does not matter.
>>>> 
>>>> There's nothing to stop "+" being extended so that xsd:dataTime + xsd:duration ->  xsd:dateTime
>>>> 
>>>> http://www.w3.org/TR/xmlschema-2/#adding-durations-to-dateTimes
>>>> 
>>>> SUM would need to say
>>>> sum(durations and at most DT) ->  DT
>>>> The result type of SUM is changed.
>>> 
>>> Good point, though that's more a good example of why overloading is a bad idea, IMHO. :)
>>> 
>>>> See also string + string.
>>> 
>>> Quite.
>>> 
>>> OK, I feel I have enough information to start drafting a response to Jeen now, I'll circulate it later today.
>>> 
>>> - Steve
>>> 
>>>> An error in an aggregation function in SELECT does not cause a row to be skipped - it becomes an unbound because of the SELECT expression.  An error in FILTER eliminates the row.
>>>> 
>>>> On 26/06/11 09:00, Steve Harris wrote:
>>>>> I think it was mostly just that at the time I wrote the def'n there was no (obvious, explicit) function for +, makes sense to change it to me.
>>>> 
>>>> True - there isn't because dispatch by operator table isn't a function.  It could be - but it currently isn't.
>>>> 
>>>> It does make the conditions for extending "+" a bit tricky though.
>>>> What conditions are needed?
>>>> 
>>>>   Andy
>>>> 
>>>>> 
>>>>> - Steve
>>>>> 
>>>>> On 2011-06-25, at 16:18, Lee Feigenbaum wrote:
>>>>> 
>>>>>> On the surface, Jeen's reasoning makes sense to me.
>>>>>> 
>>>>>> Steve, did we/you consider defining SUM instead of "+" instead of in terms of op:numeric-add?
>>>>>> 
>>>>>> Lee
>>>>>> 
>>>>>> -------- Original Message --------
>>>>>> Subject: SUM aggregate operator and non-numeric literals
>>>>>> Resent-Date: Thu, 23 Jun 2011 01:05:51 +0000
>>>>>> Resent-From: public-rdf-dawg-comments@w3.org
>>>>>> Date: Thu, 23 Jun 2011 13:05:10 +1200
>>>>>> From: Jeen Broekstra<jeen.broekstra@gmail.com>
>>>>>> To: public-rdf-dawg-comments@w3.org
>>>>>> 
>>>>>> 
>>>>>> Hi DAWG,
>>>>>> 
>>>>>> The current definition of SUM (section 18.4) is as follows :
>>>>>> 
>>>>>> ==begin quote==
>>>>>> Definition: Sum
>>>>>> numeric Sum(multiset M)
>>>>>> 
>>>>>> The Sum set function is used by the SUM aggregate in the syntax.
>>>>>> 
>>>>>> Sum(M) = Sum(ToList(Flatten(M))).
>>>>>> 
>>>>>> Sum(S) = op:numeric-add(S1, Sum(S2..n)) when card[S]>   1
>>>>>> Sum(S) = op:numeric-add(S1, 0) when card[S] = 1
>>>>>> Sum(S) = 0 when card[S] = 0
>>>>>> 
>>>>>> In this way, Sum({1, 2, 3}) = op:numeric-add(1, op:numeric-add(2,
>>>>>> op:numeric-add(3, 0))).
>>>>>> ==end quote==
>>>>>> 
>>>>>> Given that the definition of SUM is directly in terms of the
>>>>>> op:numeric-add XPath function, it follows that it can only be applied on
>>>>>> numeric literals. Therefore, any SUM that aggregates over a set of
>>>>>> values that contains a non-numeric type will result in a type error. Not
>>>>>> even an extension of the SPARQL operator table in section 17.3 will
>>>>>> help, as SUM is not defined in terms of those operators.
>>>>>> 
>>>>>> In other words, if we have the following data:
>>>>>> 
>>>>>> :a rdf:value "1" .
>>>>>> :a rdf:value "2"^^xsd:integer .
>>>>>> :b rdf:value "3"^^xsd:integer .
>>>>>> 
>>>>>> And the following query:
>>>>>> 
>>>>>> SELECT (SUM(?val) as ?value)
>>>>>> WHERE {
>>>>>>  ?a rdf:value ?val .
>>>>>> } GROUP BY ?a
>>>>>> 
>>>>>> The result will be always a type error.
>>>>>> 
>>>>>> I would argue that having the same extensibility mechanisms available
>>>>>> for SUM as we have for, for example, the + operator would be preferable.
>>>>>> That way, implementations wanting to offer a more forgiving version of
>>>>>> the SUM operator (one which silently ignores the non-numerics, for
>>>>>> example), could do so while staying spec-compliant.
>>>>>> 
>>>>>> 
>>>>>> Regards,
>>>>>> 
>>>>>> Jeen
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>> 
>>>> 
>>> 
>>> --
>>> Steve Harris, CTO, Garlik Limited
>>> 1-3 Halford Road, Richmond, TW10 6AW, UK
>>> +44 20 8439 8203  http://www.garlik.com/
>>> Registered in England and Wales 535 7233 VAT # 849 0517 11
>>> Registered office: Thames House, Portsmouth Road, Esher, Surrey, KT10 9AD
>>> 
>>> 
>> 
> 

-- 
Steve Harris, CTO, Garlik Limited
1-3 Halford Road, Richmond, TW10 6AW, UK
+44 20 8439 8203  http://www.garlik.com/
Registered in England and Wales 535 7233 VAT # 849 0517 11
Registered office: Thames House, Portsmouth Road, Esher, Surrey, KT10 9AD
Received on Monday, 27 June 2011 12:21:02 GMT

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