diff --git a/lib/src/sparql/eval.rs b/lib/src/sparql/eval.rs
index dd0e060b..c28e9ced 100644
--- a/lib/src/sparql/eval.rs
+++ b/lib/src/sparql/eval.rs
@@ -135,6 +135,27 @@ impl<S: EncodedQuadsStore> SimpleEvaluator<S> {
                     iter
                 }))
             }
+            PlanNode::Extend {
+                child,
+                position,
+                expression,
+            } => {
+                let eval = self.clone();
+                Box::new(
+                    self.eval_plan(*child, from)
+                        .filter_map(move |tuple| match tuple {
+                            Ok(mut tuple) => {
+                                put_value(
+                                    position,
+                                    eval.eval_expression(&expression, &tuple)?,
+                                    &mut tuple,
+                                );
+                                Some(Ok(tuple))
+                            }
+                            Err(error) => Some(Err(error)),
+                        }),
+                )
+            }
             PlanNode::HashDeduplicate { child } => {
                 let iter = self.eval_plan(*child, from);
                 let mut values = HashSet::with_capacity(iter.size_hint().0);
@@ -335,16 +356,17 @@ fn get_pattern_value(
 fn put_pattern_value(selector: &PatternValue, value: EncodedTerm, tuple: &mut EncodedTuple) {
     match selector {
         PatternValue::Constant(_) => (),
-        PatternValue::Variable(v) => {
-            let v = *v;
-            if tuple.len() > v {
-                tuple[v] = Some(value)
-            } else {
-                if tuple.len() < v {
-                    tuple.resize(v, None);
-                }
-                tuple.push(Some(value))
-            }
+        PatternValue::Variable(v) => put_value(*v, value, tuple),
+    }
+}
+
+fn put_value(position: usize, value: EncodedTerm, tuple: &mut EncodedTuple) {
+    if tuple.len() > position {
+        tuple[position] = Some(value)
+    } else {
+        if tuple.len() < position {
+            tuple.resize(position, None);
         }
+        tuple.push(Some(value))
     }
 }
diff --git a/lib/src/sparql/plan.rs b/lib/src/sparql/plan.rs
index fc06cd08..77cd1b71 100644
--- a/lib/src/sparql/plan.rs
+++ b/lib/src/sparql/plan.rs
@@ -26,6 +26,11 @@ pub enum PlanNode {
         entry: Box<PlanNode>,
         children: Vec<PlanNode>,
     },
+    Extend {
+        child: Box<PlanNode>,
+        position: usize,
+        expression: PlanExpression,
+    },
     HashDeduplicate {
         child: Box<PlanNode>,
     },
@@ -222,7 +227,11 @@ impl<'a, S: EncodedQuadsStore> PlanBuilder<'a, S> {
                 }
             }
             GraphPattern::Graph(g, p) => unimplemented!(),
-            GraphPattern::Extend(p, v, e) => unimplemented!(),
+            GraphPattern::Extend(p, v, e) => PlanNode::Extend {
+                child: Box::new(self.build_for_graph_pattern(p, input, variables)?),
+                position: variable_key(variables, &v),
+                expression: self.build_for_expression(e, variables)?,
+            },
             GraphPattern::Minus(a, b) => unimplemented!(),
             GraphPattern::Service(n, p, s) => unimplemented!(),
             GraphPattern::AggregateJoin(g, a) => unimplemented!(),
diff --git a/lib/tests/rdf_test_cases.rs b/lib/tests/rdf_test_cases.rs
index 95450926..62498d61 100644
--- a/lib/tests/rdf_test_cases.rs
+++ b/lib/tests/rdf_test_cases.rs
@@ -365,7 +365,7 @@ impl<'a> Iterator for TestManifest<'a> {
                             Some(Term::BlankNode(list)) => {
                                 self.manifests_to_do.extend(
                                     RdfListIterator::iter(&self.graph, list.clone().into())
-                                        .flat_map(|m| match m {
+                                        .filter_map(|m| match m {
                                             Term::NamedNode(nm) => Some(nm.as_url().clone()),
                                             _ => None,
                                         }),
diff --git a/lib/tests/sparql_test_cases.rs b/lib/tests/sparql_test_cases.rs
index ed267df8..be653dcc 100644
--- a/lib/tests/sparql_test_cases.rs
+++ b/lib/tests/sparql_test_cases.rs
@@ -481,7 +481,7 @@ impl<'a> Iterator for TestManifest<'a> {
                             Some(Term::BlankNode(list)) => {
                                 self.manifests_to_do.extend(
                                     RdfListIterator::iter(&self.graph, list.clone().into())
-                                        .flat_map(|m| match m {
+                                        .filter_map(|m| match m {
                                             Term::NamedNode(nm) => Some(nm.as_url().clone()),
                                             _ => None,
                                         }),