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Translation to Algebra

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Mon, 17 Jan 2011 18:44:17 +0000
Message-ID: <AANLkTinrvZX9MaNbSOe_E2qBtHg7VKrk9TOmEegZpg+M@mail.gmail.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
Hi all,
I am just wondering how to interpret the not yet fully complete
definition of the translation to the SPARQL algebra, but going back to
the 1.0 spec also doesn't really help :-(

I assume the query pattern
{ triple1 OPTIONAL { BGP } triple 2 }
is ok according to the grammar.

Now the question is whether optional applies to both triples or only
to the first one. In the former case I would get:
Join(LeftJoin(Bgp(triple1), Bgp(BGP), true), Bgp(triple2))
whereas in the latter case it would be:
LeftJoin(Join(Bgp(triple1), Bgp(triple2)), Bgp(BGP), true),
both already with Z simplified away.
I tend to assume the former since my understanding is that I work from
left to right, but I couldn't see that from the spec (also the 1.0
spec since this is not even 1.1 stuff and this bits in 1.1 are still
not finalized at least according to the @@ marks in the relevant
places)

Similarly, I wonder how { triple1 {} triple2 } would be translated.
triple1 {} triple2 is not a BGP/triples block (is it?), so I guess I
would have use the description for GroupGraphPattern to translate the
three elements. For the first element (triple1) that gives Join(Z,
Bgp(triple1)), where I got Bgp(triple1) from Translate(triple1) where
I treat triple1 as TriplesBlock. The next element is, however, the
empty group, for which I also would have to obtain Translate({}), but
now {} is not a TriplesBlock but a group, so I go back into the
algorithm for GroupGraphPatterns, which again requires me to compute
translate({}), etc. I think it is assumed that I implicitly take the
elements out of the group to translate them, but then I have to
explicitly do that for groups whereas I don't have to do it from
things that are already just blocks of triples. Furthermore, it is not
said AFAICT that in this case TriplesBlock translates into Z.

I have to say that the triples that are not in a group give me a bit
of a headache :-(

Thanks for any clarification,
Birte

-- 
Dr. Birte Glimm, Room 309
Computing Laboratory
Parks Road
Oxford
OX1 3QD
United Kingdom
+44 (0)1865 283520
Received on Monday, 17 January 2011 18:44:50 GMT

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