Hi all,
I’ve a question on the model query writers should have when reasoning about
their work. I think the naive view would be that adding grouping would _not_
alter the meaning of the query but this is not the case.
A first example comes from OPTIONAL. A query like
select * {
?a :ppp1 ?c .
{
optional { ?a :ppp2 ?d }
}
}
Is technically equivalent to
select * {
?a :ppp1 ?c .
?a :ppp2 ?d .
}
because the first form looks like
(join (bgp ?a :ppp1 ?c)
(left-join identity (bgp ?a :ppp2 ?d))
and the left-join will discard no solutions at run-time leaving the outer-most
join to merge the two patterns and _discard_ any ?a’s that don’t have a :ppp2
triple.
This follows pretty clearly from the SPARQL definition but also seems
surprising from a query writers perspective.
A second example occurs with BIND.
select * {
?a :ppp1 ?c .
bind( 2 * ?c as ?twiceC )
}
is _not_ the same as
select * {
?a :ppp1 ?c .
{
bind( 2 * ?c as ?twiceC )
}
}
The second form will leave ?twiceC unbound as ?c has no binding inside of the
BIND. Note that this also means that you cannot distribute common patterns out
a union because
select * {
{
?x a ?p .
?p ex:bar ?foo .
}
union
{
?x a ?p .
bind(?p as ?foo)
}
}
is _not_ the same as
select * {
?x a ?p .
{
?p ex:bar ?foo .
}
union
{
bind(?p as ?foo)
}
}
If my examples are correct, then SPARQL seems more difficult than it should be
to reason about than it should be. I’d welcome comments and thoughts.
thanks,
--
Gary Warren King, metabang.com
Cell: (413) 559 8738
Fax: (206) 338-4052
gwkkwg on Skype * garethsan on AIM * gwking on twitter
