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https://issues.apache.org/jira/browse/JENA-1519?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16437326#comment-16437326
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ASF GitHub Bot commented on JENA-1519:
--------------------------------------

Github user afs commented on a diff in the pull request:

    https://github.com/apache/jena/pull/394#discussion_r181393545
  
    --- Diff: 
jena-arq/src/main/java/org/apache/jena/sparql/algebra/OpWalker.java ---
    @@ -139,6 +139,7 @@ protected void visitN(OpN op) {
             @Override
             protected void visitExt(OpExt op) {
                 before(op) ;
    +            super.visitExt(op);
    --- End diff --
    
    Not sure about this. It is visiting both the effective op and the real 
OpExt. Shoudn't it be the visitor deciding that? ie. visiting 
`op.effectiveOp()` is required?


> OpWalkerVisitor should interact better with custom operators
> ------------------------------------------------------------
>
>                 Key: JENA-1519
>                 URL: https://issues.apache.org/jira/browse/JENA-1519
>             Project: Apache Jena
>          Issue Type: Bug
>          Components: ARQ
>    Affects Versions: Jena 3.6.0
>            Reporter: Jeremy Coulon
>            Priority: Major
>
> When visiting an OpExt, current implementation of OpWalkerVisitor is just 
> calling the visitor on opExt.effectiveOp().
> However if OpExt is replacing a sub-tree of the algebra instead of a single 
> leaf node, the sub-tree is ignored by walker.
> I found this bug when trying to execute a query with a MINUS operator with 
> either side of the operator being replaced by an custom operator.
> For example the following query is not working, if I transform both side of 
> MINUS with a custom operator:
> {{(project (?animal)}}
> {{  (minus}}
> {{    (bgp (triple ?animal rdf:type ex:Animal))}}
> {{    (filter (|| (= ?type ex:Reptile) (= ?type ex:Insect))}}
> {{      (bgp (triple ?animal rdf:type ?type)))))}}
> which I transform into:
> {{(project (?animal)}}
> {{  (minus}}
> {{    (op-ext )}}
> {{    (op-ext )))}}
> The reason is that MINUS is calling OpVars.visibleVars() which does not walk 
> into the subop of the filter.



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