SPARQL: Variables from outside group are visible inside group

[cygri]

The following query's query pattern has two group graph patterns:

SELECT * {
  { BIND ("a" AS ?a) }
  { BIND (?a AS ?b) } 
}

According to the SPARQL spec, the result of the query is defined as the result of evaluating each group independently, and then joining the results:

• Evaluating the first group should yield a single solution with ?a bound to "a".
• Evaluating the second group should yield a single solution with no variables bound. Each group is evaluated independently, and ?a is not bound within the group, so the BIND leaves ?b unbound.
• Joining both results should yield a single solution with ?a bound to "a".

In RDF4J, however, the query returns a single solution with both ?a and ?b bound to "a". This is incorrect.

Similar problems exists for other language constructs. For example, changing the second group to an OPTIONAL shows the same problem. But changing it to a subquery does not.

[abrokenjester]

Can you give an example query with an optional pattern that shows this issue? Reason I ask is that first glance the issue appears to be specific to how BIND is processed, but I may be overlooking something.

[cygri]

What I meant is, it's not just group graph patterns that are affected, but if the second group was replaced by, say, OPTIONAL { BIND (?a AS ?b) }, it shows the same problem (or so I seem to remember—haven't tried it again now).

It may well be specific to BIND.

[abrokenjester]

I have just realized that this fix is incomplete: while all the mechanics are put in place, the query planner doesn't actually use the new logic to determine what join strategy to use. 🤦‍♂️

That'll teach me for adding a fix without a regression test.