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RE: property paths

From: Polleres, Axel <axel.polleres@siemens.com>
Date: Mon, 26 Mar 2012 23:29:48 +0200
To: Gregory Williams <greg@evilfunhouse.com>, SPARQL Working Group <public-rdf-dawg@w3.org>
Message-ID: <9DA51FFE5E84464082D7A089342DEEE80137B3E9428F@ATVIES9917WMSX.ww300.siemens.net>
Hi Greg, all,

Indeed, what you write is some of the commenters argument.
However, last time we agreed on sticking - as the default - 
with the counting semantics and having a switch to turn on 
non-counting semantics on full paths (DISTINCT()) rather than 
the other way around (ALLPATHS()).

Unless you object to last week's decision, I'd prefer not to reopen the 
issue at this point and move on with the resolution from last week, 
putting proposals to default path semantics over the whole query to 
noncounting or having finer grained distinction, such as DISTINCT() 
at subpath-level to discussion for the future work items list.

We had the issue that common practice required syntax not directly 
available in the standard already in the previous addition and fixed 
that now (MINUS). So, I guess it's fair enough to proceed as we have 
resolved now on propoert paths and see how it's adopted, whereupon a 
future WG can react, as necessary, as we did with MINUS. 



> -----Original Message-----
> From: Gregory Williams [mailto:greg@evilfunhouse.com] 
> Sent: 26 March 2012 21:31
> To: SPARQL Working Group
> Subject: property paths
> Having unfortunately missed last week's call where there was 
> a resolution about property paths, I wanted to at least send 
> some thoughts on the course we seem to be taking.
> After having done some re-implementation over the last week 
> to address some bugs in my implementation, I added support 
> for distinct paths using (what I think is) the new 
> DISTINCT(path) syntax. After this work, I think I agree with 
> Andy that it's not a big implementation burden (and as noted 
> by many commenters, has some obvious benefits for 
> performance). That being said, I think I also agree with 
> Steve's concerns: I suspect my ease in implementing the 
> second path semantics is a result of not trying to have a 
> high-performance implementation. Doing optimization on 
> property paths well is probably pretty hard, and having a 
> second set of path semantics certainly doesn't help.
> Beyond the implementation burden, I'm very concerned that 
> we're rushing into this. Assuming that two path semantics are 
> required at this point (it seems we've mostly agreed on 
> this), I'm worried about the direction we seem to be taking 
> syntactically. From last week's resolution, I take it that 
> the current thinking on design is that something like { ?s 
> pathexpr ?o } will do the counting semantics (the old design) 
> and { ?s DISTINCT(pathexpr) ?o } will do the distinct 
> semantics. Was there any discussion last week about similar 
> syntax for the counting semantics? I know at one point there 
> was discussion of an ALLPATHS keyword.
> I know there was concern from some that DISTINCT() is a 
> rather wordy way of asking for the distinct semantics. I'm 
> concerned that we may end up in a situation where it turns 
> out that most people do end up wanting the distinct 
> semantics, and with our current design, we will have done the 
> Huffman coding all wrong -- distinct semantics will require a 
> long keyword, while the counting semantics won't. An 
> alternative I had thought about was having keywords for both 
> semantics (e.g. DISTINCT and ALLPATHS), and leaving the 
> choice of semantics for queries not specifying a keyword to 
> either the implementation or a future WG. I think that might 
> make some implementors happy, at the expense of result 
> cardinality differences between implementations in the near 
> term (that is, until a future WG might nail down which 
> semantics should be the default).
> thanks,
> .greg
Received on Monday, 26 March 2012 21:30:18 UTC

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