LATERAL query evaluation error

[afs]

Version

4.10.0, 5.0.0 development

What happened?

Details w3c/sparql-dev#100 (comment)

Query:

PREFIX ex: <http://example.org/>

SELECT ?s ?o WHERE {
    ?s a ex:T.
    OPTIONAL { LATERAL {SELECT ?s ?o WHERE { ?s ex:p ?o } ORDER BY ?o LIMIT 2} }

Data:

PREFIX ex: <http://example.org/>

ex:s1 a ex:T ;
   ex:p "11" , "12" , "13" .

ex:s2 a ex:T ;
   ex:p "21" , "22" , "23" .

ex:s3 a ex:T .

Correct result:

----------------
| s     | o    |
================
| ex:s3 |      |
| ex:s1 | "11" |
| ex:s1 | "12" |
| ex:s2 |      |
----------------

Wrong result:

----------------
| s     | o    |
================
| ex:s3 |      |
| ex:s1 | "11" |
| ex:s1 | "12" |
| ex:s2 | "21" |
| ex:s2 | "22" |
----------------

[kinow]

Strange, I would have guess the query would return the wrong result (haven't used a lateral join in a while, last I used was in Postgres to perform like a for-each).

[afs]

After staring at the query and execution for ages ... the "wrong" result now looks right to me
It is for completely the wrong reasons though.

Discussion continues on
w3c/sparql-dev#100 (comment)
(this is only part of that issue).

[afs]

Repurposing this issue: the lateral implementation generally can improved. The jena4.x implementation makes assumptions about how ARQ executes and these assumptions aren't always valid.