Re: Draft Response for "Comments on SPARQL 1.1 WD 20091022"

On 6 Nov 2009, at 13:19, Andy Seaborne wrote:

> 2009/11/6 Steve Harris <steve.harris@garlik.com>:
>> On 6 Nov 2009, at 08:58, Andy Seaborne wrote:
>>
>>> Proposed response:
>>>
>>> http://www.w3.org/2009/sparql/wiki/CommentResponse:ldodds-query-1
>>
>> Re. FILTER( EXISTS { ... })
>>
>> I think that at the F2F that agreed that we wouldn't allow for  
>> EXISTS inside
>> FILTER for this reason, and because there was some reluctance to  
>> have BGP
>> expressions in the FILTER syntax.
>>
>> If that matches the minutes, and/or other people's recollection it  
>> might be
>> worth pointing that out, as it answers the concern.
>>
>
> I wasn't there and I'll wait for the formal outcomes but I read the
> logs as discussing sub-SELECT in FILTER.  That does make sense because
> (1) we have named variables so it's a lot less necessary to be able to
> put a scalar sub-SELECT into a FILTER for testing and (2) FILTER
> motion can complicate the scoping rules.

I recall it as all sub-queries (including FILTER(EXISTS{}) as a sub- 
ASK), but with jetlag and all I wouldn't swear to it!

> Neither of these apply for EXISTS.  On (1), the best I came up with is
>
> { SELECT (count(*) > 0 AS ?boolean)
>   WHERE { pattern1 . EXISTS{pattern2} }
> }
>
> which I think we'd agree is "inconvenient" :-)  and inefficient.
>
> { SELECT (count(*) > 0 AS ?boolean)
>   WHERE { SELECT * { pattern1 . EXISTS{pattern2} } LIMIT 1 }
> }
>
> is faster and worse to write.

"Faster" presumes something about the optimiser, but I'll grant that  
it's easier to optimise.

The feeling was that it didn't add any capability, has the coping  
peculiarity and messes up the grammar. Also, it's a bit of a corner- 
case requirement. There could be a convincing use-case, but I don't  
think anyone came up with one at the F2F.

- Steve

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Received on Friday, 6 November 2009 16:58:31 UTC