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Re: rq25 aggregate functions

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Tue, 22 Dec 2009 11:24:01 +0000
Message-ID: <4B30AC51.4000607@talis.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
Can we raise an ISSUE for this so it does not get lost, please?

	Andy

On 21/12/2009 23:56, Steve Harris wrote:
> On 21 Dec 2009, at 18:07, Andy Seaborne wrote:
>
>> Steve and I were discussing aggregates on IRC: this is for the record
>> because we didn't have time to complete the discussion there and then.
>>
>> What we both want is that
>>
>> sum(1, unbound) -> 1
>>
>> sum(1, 2/0) -> error
>>
>> i.e skip unbounds in the group but return error on bad expressions.
>>
>> COALESCE does not help because it treats both as the same. And does
>> not work for min and max which are multi-datatype.
>>
>> The key text is:
>>
>> [[
>> Definition: Aggregation
>>
>> Any unbound values exp · μ' do not appear in the resulting value multiset
>> ]]
>>
>> which (my reading) that says evaluate exp on μ' then see if it's unbound.
>>
>> But the single variable case sum(?x) has an expression of ?x so exp ·
>> μ' is called on unbound ?x (which is an error) then the test is made
>> to see if it's unbound (it's not - it's now error).
>>
>> We could special-case evaluation of aggregates. If it's a single
>> variable, then return value or unbound. If it's any other expression
>> (including ?x+0), then evaluate as normal, i.e. error. There are
>> otherways for sum (e.g. the value of unbound is always zero) but is
>> using the special feature that zero is the identity value for + so
>> does not work for other aggregates.
>
> Another option would be to say that if there are any variables appearing
> in exp (other than as an argument to COALESCE) that are unbound in μ',
> the result in unbound.
>
> It's unfortunate for aggregates that eval(?x) for unbound ?x is an error.
>
> - Steve
>
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Received on Tuesday, 22 December 2009 11:24:20 GMT

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