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Re: Review of Query document

From: Steve Harris <steve.harris@garlik.com>
Date: Mon, 27 Sep 2010 13:55:28 +0100
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>, Andy Seaborne <andy.seaborne@epimorphics.com>
Message-Id: <5DB4D80A-8AF1-49FC-BA64-BA52864F61AE@garlik.com>
To: Gregory Williams <greg@evilfunhouse.com>
Thanks Greg.

On 2010-09-20, at 23:18, Gregory Williams wrote:
>>> 10 Aggregates
> This section should mention somewhere that aggregates must be aliased in order to project them. A brief mention with a link to 15.1.2 SELECT expressions would be sufficient.

Yep, will do.

>>>   10.1 Aggregate Example
> The introductory text seems a bit thin for readers that may not already be familiar with aggregates. Similarly, the example in 10.1 might be aided by some explanatory text detailing how the final result is arrived at.


>>>   10.2 Algebra Operators
> "Aggregation, a function which calculates a scalar value as an output of the aggregate expression in the SELECT clause." -- aggregate expressions can also be in the HAVING clause, right (not just SELECT)?

Not as written, but it should, yes.

> The 'scalar' argument to Aggregation() is said to be a set, but in the example Aggregation() is called with a value of 0.

Yes, that got missed when I changed scalar, will fix.

> The second argument to the call to the aggregate set function 'func' is defined as card[range(g)] - card[M], but the use of this value isn't discussed until later in 10.2.2, and then only vaguely. It is never used in the definitions of the set functions.

It's not needed by the provided functions, but I think someone (Andy?) had some argument as to why it was useful to extension functions.

>>>       10.2.1 HAVING
>>>       10.2.2 Set Functions
> Just before the definition of Sum, the example says the result should be "6.0 (decimal)", but it should be "6.0 (float)".

Right, I must have done the promotion wrong in my head.

> In the definition of GroupConcat, "unicode codepoint 32" is probably better described as "unicode codepoint U+0020".
> GroupConcat(S, scalar) is defined in terms of fn:string-join, but that function is never defined or referenced again. The fn prefix is defined in section 1.2.1, but since this function is never discussed in the document, it should probably be hyperlinked to the xpath definition at <http://www.w3.org/2005/xpath-functions/#string-join>.


>>> Material to move to formal definitions:
>>>       10.2.3 Mapping from Abstract Syntax to Algebra
> The example says the SUM expression "becomes Aggregation((?a), (?val), Sum, (), BGP(?a rdf:value ?val))." This form of Aggregation() uses one more argument than the definition in 10.2 (the GROUP BY variable). I would have expected the SUM expression to become
> Aggregation((?val), Sum, (), Group((?a), BGP(?a rdf:value ?val)))

Thanks, this was left over from an earlier defn.

> In the "Joining Aggregate Values" section, I have no idea what the introductory sentence is meant to convey. I may have misunderstood the definition of AggregateJoin, but it seems like it will produce a multiset of single-mapping sets, {agg_i -> range(A_i)}. I would have expected something like:
> AggregateJoin(A) = { { (agg_i -> range(A_i)) | dom(A_i) = k } | k in set-union(dom(A)) }
> The algorithmic sketch of using AggregateJoin in 17.2.3 might be more intuitive if there were more than one 'Let A_i' line (more than one aggregate operation in the query).

I'll have to come back to this, as I'm not sufficiently swapped in on this to be able to disentangle it.

>>> 11 Subqueries
> There needs to be introductory text for subqueries.
> "It is an error to reuse variable names both inside and outside a subquery when the variable is not projected from the subquery." -- I checked with Andy, and he said of this: "It's wrong and (for composition reasons) we decided otherwise."


- Steve

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Received on Monday, 27 September 2010 12:56:05 UTC

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