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https://issues.apache.org/jira/browse/JENA-1468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16343243#comment-16343243
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ASF GitHub Bot commented on JENA-1468:
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Github user afs commented on the issue:
https://github.com/apache/jena/pull/345
It does not need to change. It will behave as before.
The changes here differentiate between a table that is of class
`TableEmpty`, using function `TableEmpty.isTableEmpty`, and a table that
evaluates to empty (no rows) using method `Table.isEmpty`. Previously, there
was only the latter method.
`VALUES ?foo {}` creates algebra `TableN` with declared variable `?foo` and
no rows. It printed (incorrectly) as `(table empty)` but internally it was a
`TableN` and method `isEmpty` is true (no rows).
Now it prints as `(table (vars ?foo))`. There is no change to the data
structure in the algebra.
The difference is on conversion to a string, and on `.equals`, then only if
read in again because it was not round-tripping the data structure, even if it
evaluated to the same thing. Declared variables make no difference to the value
of a table and that follows the SPARQL 1.1 spec exactly.
As it makes no eval difference, we haven't noticed before.
> Empty VALUES clauses lose variable bindings when compiled to a SSE
> ------------------------------------------------------------------
>
> Key: JENA-1468
> URL: https://issues.apache.org/jira/browse/JENA-1468
> Project: Apache Jena
> Issue Type: Bug
> Components: ARQ
> Affects Versions: Jena 3.1.1, Jena 3.2.0, Jena 3.3.0, Jena 3.4.0, Jena
> 3.5.0, Jena 3.6.0
> Reporter: Rick
> Priority: Major
> Labels: SPARQL, sse
>
> Given a SPARQL query like this:
> SELECT ?foo WHERE {
> ?foo ?bar ?baz . # Not essential for problem just to show we can't
> optimise it away.
> VALUES ?foo { }
> }
> If you compile it into an SSE with something like this:
> {{Algebra.compile(QueryFactory.create("SELECT ?foo WHERE \{ ?foo ?bar ?baz .
> VALUES ?foo { }}"));}}
> You end up with a semantically different SSE:
> {{(project (?foo)}}
> {{ (join}}
> {{ (bgp (triple ?foo ?bar ?baz))}}
> {{ (table empty)))}}
> Note how "(table empty)" has lost the ?foo binding, which means that this
> query returns all results for ?foo, where it should return no results.
> I'm not sure how this query should be expressed in SSE form I'd suggest:
> {{(table (vars ?foo) empty)}}
> Though currently this will get simplified in [code like
> this|https://github.com/apache/jena/blob/94eb3fc99ba1ff3991a629fffa6e6cf8b52d6c53/jena-arq/src/main/java/org/apache/jena/sparql/sse/builders/BuilderTable.java#L55-L64]
> into an EmptyTable if you do something like:
> {{BuilderTable.build(Item.createList((SSE.parse("(table (vars ?foo)
> empty)")));}}
> Additionally it appears quite a bit of code carries through this assumption
> that an [empty table will never have a
> binding|https://github.com/apache/jena/blob/94eb3fc99ba1ff3991a629fffa6e6cf8b52d6c53/jena-arq/src/main/java/org/apache/jena/sparql/algebra/table/TableEmpty.java#L57].
> As far as I can tell this issue effects many prior versions of JENA including
> 3.6.0.
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