fix: bug with `SELECT *` inside `SELECT *` (#2190)

Fixing use of `SELECT *` in sub-select within `SELECT *` parent query as discovered in https://github.com/RDFLib/rdflib/issues/1722.

Now when an instance of `SELECT *` is encountered, the query tree/plan builder now correctly considers the projected variables of any sub-select statements when deciding which variables should be projected out. 

Fixes <https://github.com/RDFLib/rdflib/issues/1722>.

1 files changed, 23 insertions, 0 deletions

diff --git a/rdflib/plugins/sparql/algebra.py b/rdflib/plugins/sparql/algebra.py
index d538920e..923957b2 100644
--- a/rdflib/plugins/sparql/algebra.py
+++ b/rdflib/plugins/sparql/algebra.py
@@ -511,6 +511,7 @@ def _findVars(x, res: Set[Variable]) -> Optional[CompValue]:  # type: ignore[ret
         elif x.name == "SubSelect":
             if x.projection:
                 res.update(v.var or v.evar for v in x.projection)
+
             return x
 
 
@@ -637,6 +638,7 @@ def translate(q: CompValue) -> Tuple[CompValue, List[Variable]]:
     traverse(q.where, functools.partial(_findVars, res=VS))
 
     # all query types have a where part
+    # depth-first recursive generation of mapped query tree
     M = translateGroupGraphPattern(q.where)
 
     aggregate = False
@@ -679,6 +681,12 @@ def translate(q: CompValue) -> Tuple[CompValue, List[Variable]]:
 
     if not q.projection:
         # select *
+
+        # Find the first child projection in each branch of the mapped query tree,
+        # then include the variables it projects out in our projected variables.
+        for child_projection in _find_first_child_projections(M):
+            VS |= set(child_projection.PV)
+
         PV = list(VS)
     else:
         PV = list()
@@ -731,6 +739,21 @@ def translate(q: CompValue) -> Tuple[CompValue, List[Variable]]:
     return M, PV
 
 
+def _find_first_child_projections(M: CompValue) -> Iterable[CompValue]:
+    """
+    Recursively find the first child instance of a Projection operation in each of
+      the branches of the query execution plan/tree.
+    """
+
+    for child_op in M.values():
+        if isinstance(child_op, CompValue):
+            if child_op.name == "Project":
+                yield child_op
+            else:
+                for child_projection in _find_first_child_projections(child_op):
+                    yield child_projection
+
+
 # type error: Missing return statement
 def simplify(n: Any) -> Optional[CompValue]:  # type: ignore[return]
     """Remove joins to empty BGPs"""