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https://issues.apache.org/jira/browse/JENA-763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14094238#comment-14094238
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Rob Vesse commented on JENA-763:
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Another complex case where doing quad form transform first doesn't work is when
property paths are used. The optimiser will flatten simple paths out into BGPs
which mean we still have to re-apply quad form transform later as well as
otherwise we get an algebra that mixes {{quadpattern}} and {{bgp}}.
If you mix in sub-queries with our custom modifiers involved we get complex
queries where applying quad form transform during algebra generation and after
optimisations still leads to an incorrect query.
> 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
>
> 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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