unexpected SPARQL playground answer (VALUES and OPTIONAL)

Question

Consider the following SPARQL query:

select * where
{
  values ?x { 1 2 3 } .
  optional { values (?x ?xp) { (2 2) (3 3) (4 4) } }
}

I would think it returns the following:

?x ?xp
------
1  NULL
2  2
3  3

However, the "SPARQL playground" http://sparql-playground.isb-sib.ch/ returns the following:

?x ?xp
------
2  2
3  3
4  4

Can somebody explain this?

Answer 1

事实上这是一个但由于芝麻引擎。 目前的引擎，Sesame 2.8.6解决了这个问题。