This is blocking an implementation I am doing at Syapse, so I set the priority to high.
I found an apparent bug in the blazegraph implementation of OPTIONAL when "/" is used. It is reproducible with the foaf kb used in http://www.w3.org/TR/2013/REC-sparql11-update-20130321/
Start by inserting these triples:
Alice and Bob know each other. Alice has two email addresses. Bob doesn't have any. (This is slightly extended from the example of OPTIONAL in the document.)
This should return the email address of a friend, or a blank if there is no email.
In this case OPTIONAL works as expected. Bob knows Alice, and Alice has two emails. Alice knows Bob and Bob does not have an email.
Now replace "?me . ?me" with "/". This should be the same except it does not bind a variable.
But now only the first two lines are returned. The line for Alice is omitted.