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https://issues.apache.org/jira/browse/JENA-763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14098576#comment-14098576
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Andy Seaborne edited comment on JENA-763 at 8/15/14 2:43 PM:
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One solution is to add {{CLUSTER BY}} to the ARQ SPARQL-extended language. The
proposal looks interesting and, aside from specific details (like the way the
{{USING}} clause is done), looks general. Coping with arbitrary syntax
extensions while still providing a framework is tricky with a need to define
the implications of the extensions all the way down the optimizer and execution
processes.
The quads transform is unique in the way it manages state. In your example, I
guess that the {{OpExt}} itself does not contain arbitrary {{GRAPH}} patterns
below it so access to the Pusher and Popper aren't needed.
There might be other ways to have transform-specific state so turning it into a
general mechanism looks hard. State can already be passed to the {{OpExt}} via
{{OpExt.apply(transform)}} with a transform that carries the state over.
Changing it to {{OpExt.apply(transform, object)}} is clearer though.
Solving specifically for quad transformation looks doable. If {{OpExt}} could
expose any real sub Ops (not {{effectiveOp}}) then the transformation walker
could go through {{OpExt}}.
was (Author: andy.seaborne):
One solution is to add {{CLUSTER BY}} to the ARQ SPARQL-extended language. The
proposal looks interesting and, aside from specific details (like the way the
{{USING}} clause is done), looks general. Coping with arbitrary syntax
extensions while still providing a framework is tricky with a need to define
the implications of the extensions all the way down the optimizer and execution
processes.
The quads transform is unique in the way it manages state. In your example, I
guess that the {{OpExt}} itself does not contain arbitrary {{GRAPH}} patterns
below it so access to the Pusher and Popper aren't needed.
There might be other ways to have transform-specific state so turning it into a
general mechanism looks hard. State can already be passed to a the {{OpExt}}
via {{OpExt.apply(transform)}} with a transform that carries the state over.
Changing it to {{OpExt.apply(transform, object)}} is clearer though.
Solving specificly for quad transformation looks doable.
> 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
> Attachments: Jena673.java
>
>
> 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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