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RE: Semantics of aggregates

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Tue, 1 Sep 2009 13:10:52 +0000
To: Steve Harris <steve.harris@garlik.com>
CC: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-ID: <B6CF1054FDC8B845BF93A6645D19BEA3693CEC2814@GVW1118EXC.americas.hpqcorp.net>


> -----Original Message-----
> From: Steve Harris [mailto:steve.harris@garlik.com]
> Sent: 1 September 2009 10:22
> To: Seaborne, Andy
> Cc: public-rdf-dawg@w3.org Group
> Subject: Re: Semantics of aggregates
> 
> On 31 Aug 2009, at 23:25, Seaborne, Andy wrote:
> 
> [snip]
> 
> >> What would be the expected behaviour given
> >>   SELECT min(?x) min(?y) min(?z) { ... }
> >> where x, y, and z each take some subset of numbers, dates etc? Also
> >> unknown datatypes pose a problem.
> >
> > I don't know (for several of the designs).
> >
> > We have to have a design that copes with the situation of mixed sets
> > of numbers although I hope it's more of a corner case to be dealt
> > with rather than a driver for the overall design.  (FWIW I value
> > consistency of results across implementations and also not having
> > errors during query evaluation aborting the overall query.)
> 
> Agreed, but I also value a solution that gives the most helpful
> results for common usecases,

My idea of a common case is when all the rows grouped have the same type or are undefined.  Is that your common case?

> though obviously that's something that's
> extremely hard (potentially impossible) to define.
> 
> > Unknown datatypes are certainly a problem as well.
> 
> Yes.
> 
> I think I'm currently leaning towards a solution that handles this
> problem more like a cast or datatype restriction. It seems like the
> easiest thing to understand that produces a single value from the
> aggregate function, which seems like it will make it easier to
> understand from the users point of view.
> 
> That said, we've not implemented many aggregates yet, so we don't have
> much experience of working with them.
> 
> - Steve

I'm not sure what the cast proposal is.  A cast isn't a datatype restriction:

Min(xsd:integer(?x))

would turn strings into integers, possibly with errors for some strings.  It will fail on a double.  But casting to a xsd:double means the result is a double even if it's all integers which I find confusing.

If it's the form:

MIN(xsd:integer, ?x)

we could have that as a restriction to the value space of xsd:integer.

	Andy


> 
> --
> Steve Harris
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Received on Tuesday, 1 September 2009 13:24:34 GMT

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