[GitHub] [jena] rvesse commented on a diff in pull request #1620: Allow toggling extended path flattening on

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rvesse commented on code in PR #1620:
URL: https://github.com/apache/jena/pull/1620#discussion_r1025030591


##########
jena-arq/src/main/java/org/apache/jena/sparql/algebra/optimize/TransformPathFlatternStd.java:
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@@ -35,8 +35,15 @@
 
 /** The path transformation step exactly as per the SPARQL 1.1 spec.
  *  i.e. joins triples rather creating BGPs.
+ *  <p>
  *  It does not produce very nice execution structures so ARQ uses
- *  a functional equivalent, but different, transformation.
+ *  a functional equivalent, but different, transformation, see {@link TransformPathFlattern}
+ *  </p>
+ *  <p>
+ *  However for users who are using property paths in their queries heavily there may be benefits to using this
+ *  transform over the simpler one.  The {@link org.apache.jena.query.ARQ#optPathFlattenExtended} symbol can be set in
+ *  an ARQ context to enable this transform in preference to the simpler transform.
+ *  </p>

Review Comment:
   Interestingly the `:p{2}` case actually looks to be expanded the wrong way round (at least to my eye) in the current code.  It produces the following:
   
   ```
   (prefix ((: <http://example/>))
     (sequence
       (path ??P0 (pathN* :p) :T1)
       (bgp
         (triple ?x :p ??P1)
         (triple ??P1 :p ??P0)
       )))
   ```
   
   When I'd expect to see the `bgp` first followed by the `path` operator



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