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https://issues.apache.org/jira/browse/JENA-763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14094363#comment-14094363
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Andy Seaborne commented on JENA-763:
------------------------------------
Could we experiment in a branch rather than experimenting on trunk? I for one
don't yet understand the issue and haven't had time to fully understand your
latest comments. This talking indirectly about custom code is not easy to
understand what is going on. Can you provide a public simulation we discuss?
Please can you provide a example for sq01 because it works fine for me:
{noformat}
(project (?x ?p)
(graph ?g
(bgp (triple ?x ?p ?y))))
{noformat}
which becomes:
{noformat}
(project (?x ?p)
(quadpattern (quad ?g ?x ?p ?y)))
{noformat}
and that looks fine. Executing the quad form on the test data gives the same
results as the test.
bq. The major complication comes in when there are sub-queries involved because
we found that you cannot apply the quad form transformation inside the sub-query
Are you applying the quad transformation on part of a query? I don't understand
why and, without knowing your case, would have thought it was not necessary.
What's the information flow needed to pass state to the partial quad transform
from whatever is driving the overall process? What about OpLabel?
I don't understand why you should have to override
{{Algebra.compileModifiers()}} which is working on syntax. The algebra coming
out of that step is sufficient (it has all the information of a query), and
quad form transform does not remove the modifiers (which are subclasses of
{{OpModifier}}) - it's even reversible. So compile, then make the quad form,
then customize.
The principle is that the algebra expression is complete information. For me,
that is the general solution.
> Transforms should interact better with custom operators
> -------------------------------------------------------
>
> Key: JENA-763
> URL: https://issues.apache.org/jira/browse/JENA-763
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ
> Affects Versions: Jena 2.12.0
> Reporter: Rob Vesse
> Assignee: Rob Vesse
>
> As already discussed briefly on the mailing list thread How to safely apply
> transforms to custom algebra operators?
> (http://s.apache.org/custom-algebra-transform) making some transforms pass
> correctly through custom algebra operators.
> {{TransformCopy}} defers the {{copy(OpExt ext)}} implementation back to the
> {{apply()}} method of {{OpExt}} which means a custom operator can do
> something simple like the following:
> {noformat}
> @Override
> public Op apply(Transform transform)
> {
> // This is required in order to not block optimizations
> return new CustomOperator(Transformer.transform(transform,
> this.subOp), this.customParams);
> }
> {noformat}
> Which will work correctly for stateless transforms but fails for transforms
> like {{Algebra.toQuadForm()}} which rely on external state. In the specific
> case of quad form transformation the external state is tracked by before and
> after visitors that are applied as the {{ApplyTransformVisitor}} works down
> the algebra with the state being used by the actual transform as it comes
> back up the algebra. However when passed through a custom operator there is
> no way to pass through the external state trackers and so inside the custom
> operator the transform may be accessing incorrect state.
> There are a couple of options for fixing this:
> # Fix this specific case by rewriting the quad form transform such that it
> does not rely on external state tracking (not sure that this is even feasible)
> # Revise the API for transforming {{OpExt}} so external state can also be
> passed where necessary
> Both options have difficulties and it may be possible to make simpler changes
> that allow the specific case of quad form transformations to be fixed without
> changing the public API.
> Another approach would be to have the quad form transform be a public class
> and provided public accessors to its external state such that a custom
> operator could specifically recognise it and special case it such that the
> external state tracking was passed onwards. More generally perhaps a marker
> interface {{StatefulTransform}} could be added which would provide a standard
> way to recognise transforms that may have this problem and provide access to
> the state trackers necessary to pass these through custom operators
> correctly. Additionally there could be overloads of
> {{Transformer.transform(Transform)}} i.e.
> {{Transformer.transform(StatefulTransform)}} that would wire things up
> appropriately allowing the existing basic approach for custom operators
> outlined above continue to work without special cases.
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