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Re: Nested Aggregate Expressions

From: Steve Harris <steve.harris@garlik.com>
Date: Tue, 5 Jun 2012 06:50:12 -0700
Cc: public-rdf-dawg@w3.org
Message-Id: <A105A009-4962-4A88-A3F2-242C6CE03123@garlik.com>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
On 5 Jun 2012, at 03:42, Andy Seaborne wrote:
> 
> On 04/06/12 23:45, Steve Harris wrote:
>> On 3 Jun 2012, at 16:03, Andy Seaborne wrote:
>> 
>>> Can aggregate expressions contain aggregates?
>>> 
>>> This comes from a recent email [1] and while these may well not actually address the application goal (a subquery was meant - see thread), the SPARQL 1.1 Query spec does permit these unusual queries.
>>> 
>>> Example:
>>> 
>>> PREFIX ex:<http://example.org/meals#>
>>> SELECT (AVG(?mealPrice * (1.0 + MAX( ?mealTip / ?mealPrice)))
>>>           AS ?avgCostWithBestTip)
>>> WHERE {
>>>  ?description ex:mealPrice ?mealPrice .
>>>  ?description ex:mealTip ?mealTip .
>>> } GROUP BY ?description
>>> 
>>> i.e.an aggregate inside an aggregate:  AVG(?x * MAX (?y) )
>>> 
>>> 
>>> One line of argument is that the expression inside the aggregate is applied to each row, so only row variables should be considered in-scope.  The aggregate AVG(max(?x)+1) is violating that as max(?x) is not a per-row expression.
> 
> (Birte) - yes this needs clarifying if we wish to rule it out, and possible even if we don't.
> 
> As the spec stands, I *think* it says its not allowed:
> 
> [[
> Definition Group:
> 
> Group evaluates a list of expressions against a solution sequence
> ...
> ]]
> 
> and the solution sequence is the grouped patterns, not after aggregation or select expressions.
> 
> [[
> Definition: Aggregation
> ]]
> talks about applying the aggregate to the solution sequences collected into a map of key to multiset.
> 
> i.e. the aggregate is evaluated over the pattern, not other aggregates and not select expressions
> 
> Steve - opinion?

Yes, I think that the definition says that you wont get a result, but it could probably be clearer about what happens, error / unbound - or do you think it's sufficiently clear?

>>> What ARQ does is to calculate the aggregates of a group as the group streams past; it does not wait until the end of evaluation of the whole block when all the elements of all the groups are known.
>>> 
>>> 
>>> Related to this is the interaction with select expressions:
>>> 
>>> SELECT (max(?x) As ?M) (avg(?M+1) AS ?A)
>>> 
>>> because the select expression rules say you can use ?M inside AVG().
>>> 
>>> If we wish to forbid this, we can do it quite easily by having a parser rule that aggregates can't appear in expression for the aggregate, which is a simple static check.
>> 
>> Oh boy, it's certainly wacky.
>> 
>> That parse rule wouldn't rule out the use of ?M above though anyway, would it?
> 
> Complicated :-)
> 
> As I read the spec, the ?Ms are different.
> 
> (max(?x) As ?M) -- select expression
> 
> avg(?M+1) -- undefined variable in the grouped pattern that is never mentioned or bound.
> 
> Like writing
> 
> avg(?noSuchVariable+1)

Right.

> Does any one have a use case that suggests it should be legal?

I don't see how it can really have any useful results.

- Steve

-- 
Steve Harris, CTO
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Received on Tuesday, 5 June 2012 13:51:06 GMT

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