On 23/02/2021 15:44, james anderson wrote:
On 2021-02-23, at 15:10:23, Andy Seaborne <a...@apache.org> wrote:
On 22/02/2021 15:31, james anderson wrote:
why did the authors exclude path variables when considering the alternatives.
this formulation appears intuitive.
ASK { <x> ?p+ <y> }
Which is almost:
ASK { <x> (!<>)+ <y> }
but for that it should yield bindings.
ASK does not yield bindings so the rewrite is valid.
And only because it's ASK.
Which is what you asked!
why was it rejected?
Time mostly.
discussions appear here and elsewhere as to the wildcard predicate expression
and its variants.
why was it not permitted as a direct expression?
ASK is not special in its evaluation of patterns.
In SELECT and more general path settings:
What would the answer be? And how many? What if there are DAGs?
why would one not interpret it analogous to
{ ?z <z>+ ?y }
What is the analogy?
(Did you mean variables in S and O?)
How many rows are you expecting to { <x> ?p+ <y> }
One? if so, what about "?p*" and "?p?" ?
Many? But there are different ways of "many" in a graph with DAGs and
SPARQL distinct connectivity semantics.
that is, as yielding successive bindings which correspond to the progression
through the graph.
The specs uses "ALP" (which is not join based) which is an algorithm for
connectivity without presuming which route is taken to encountered a
node. That affects cardinality.
If the route is required, there are sometimes multiple routes. DAGs -
which occur in e.g. RDFS quite naturally.
The common user request is path length or minimum path length, more
common than ?p* effects.
when implemented in terms of the known approach with joins, the questions would
be if a given initial solution fixes successive joins to just that respective
predicate binding or if the successive joins are over the full matched set of
predicates and, in the first case, whether they are distinct or enumerated.
What about the more general path?
(?p1/<property>/?p2)*
(?p1*/?p2*)
This seems to be a complex requirement with no natural single
definition. Certain special cases look like they some natural
definitions but each has to considered separately and specially called out.
Andy
---
james anderson | ja...@dydra.com | http://dydra.com
