diff --git a/src/engine/GroupBy.cpp b/src/engine/GroupBy.cpp
index 58428a5f3a..cc43379d9c 100644
--- a/src/engine/GroupBy.cpp
+++ b/src/engine/GroupBy.cpp
@@ -366,25 +366,23 @@ ProtoResult GroupBy::computeResult(bool requestLaziness) {
   }
 
   if (useHashMapOptimization) {
-    if (subresult->isFullyMaterialized()) {
-      using Pair = std::pair<std::reference_wrapper<const IdTable>, LocalVocab>;
-      auto gen = [](const Result& input) -> cppcoro::generator<Pair> {
-        Pair p{input.idTable(), input.getCopyOfLocalVocab()};
-        co_yield p;
-      }(*subresult);
+    auto computeWithHashMap = [this, &metadataForUnsequentialData,
+                               &groupByCols](auto&& subresults) {
       auto doCompute = [&]<int NumCols> {
         return computeGroupByForHashMapOptimization<NumCols>(
-            metadataForUnsequentialData->aggregateAliases_, std::move(gen),
+            metadataForUnsequentialData->aggregateAliases_, AD_FWD(subresults),
             groupByCols);
       };
       return ad_utility::callFixedSize(groupByCols.size(), doCompute);
+    };
+
+    if (subresult->isFullyMaterialized()) {
+      // `computeWithHashMap` takes a range, so we artificially create one with
+      // a single input.
+      return computeWithHashMap(std::array{std::pair{
+          std::ref(subresult->idTable()), subresult->getCopyOfLocalVocab()}});
     } else {
-      auto doCompute = [&]<int NumCols> {
-        return computeGroupByForHashMapOptimization<NumCols>(
-            metadataForUnsequentialData->aggregateAliases_,
-            std::move(subresult->idTables()), groupByCols);
-      };
-      return ad_utility::callFixedSize(groupByCols.size(), doCompute);
+      return computeWithHashMap(std::move(subresult->idTables()));
     }
   }
 
@@ -1512,6 +1510,8 @@ Result GroupBy::computeGroupByForHashMapOptimization(
       getExecutionContext()->getAllocator(), aggregateAliases,
       columnIndices.size());
 
+  ad_utility::Timer lookupTimer{ad_utility::Timer::Stopped};
+  ad_utility::Timer aggregationTimer{ad_utility::Timer::Stopped};
   for (const auto& [inputTableRef, inputLocalVocab] : subresults) {
     const IdTable& inputTable = inputTableRef;
     localVocab.mergeWith(std::span{&inputLocalVocab, 1});
@@ -1525,8 +1525,6 @@ Result GroupBy::computeGroupByForHashMapOptimization(
         _groupByVariables.begin(), _groupByVariables.end()};
     evaluationContext._isPartOfGroupBy = true;
 
-    ad_utility::Timer lookupTimer{ad_utility::Timer::Stopped};
-    ad_utility::Timer aggregationTimer{ad_utility::Timer::Stopped};
     for (size_t i = 0; i < inputTable.size();
          i += GROUP_BY_HASH_MAP_BLOCK_SIZE) {
       checkCancellation();
@@ -1572,10 +1570,10 @@ Result GroupBy::computeGroupByForHashMapOptimization(
       }
       aggregationTimer.stop();
     }
-    runtimeInfo().addDetail("timeMapLookup", lookupTimer.msecs());
-    runtimeInfo().addDetail("timeAggregation", aggregationTimer.msecs());
   }
 
+  runtimeInfo().addDetail("timeMapLookup", lookupTimer.msecs());
+  runtimeInfo().addDetail("timeAggregation", aggregationTimer.msecs());
   IdTable resultTable =
       createResultFromHashMap(aggregationData, aggregateAliases, &localVocab);
   return {std::move(resultTable), resultSortedOn(), std::move(localVocab)};
diff --git a/test/GroupByTest.cpp b/test/GroupByTest.cpp
index 0c2e314e46..5aa0b1bc1b 100644
--- a/test/GroupByTest.cpp
+++ b/test/GroupByTest.cpp
@@ -3,6 +3,7 @@
 // Authors: Florian Kramer (florian.kramer@mail.uni-freiburg.de)
 //          Johannes Kalmbach (kalmbach@cs.uni-freiburg.de)
 
+#include <engine/SpatialJoinAlgorithms.h>
 #include <gmock/gmock.h>
 
 #include <cstdio>
@@ -36,6 +37,7 @@ using ::testing::Optional;
 
 namespace {
 auto I = IntId;
+auto D = DoubleId;
 
 // Return a matcher that checks, whether a given `std::optional<IdTable` has a
 // value and that value is equal to `makeIdTableFromVector(table)`.
@@ -747,6 +749,47 @@ TEST_F(GroupByOptimizations, correctResultForHashMapOptimization) {
             resultWithoutOptimization->asDebugString());
 }
 
+// _____________________________________________________________________________
+TEST_F(GroupByOptimizations, hashMapOptimizationLazyAndMaterializedInputs) {
+  /* Setup query:
+  SELECT ?x (AVG(?y) as ?avg) WHERE {
+    # explicitly defined subresult.
+  } GROUP BY ?x
+ */
+  // Setup three unsorted input blocks. The first column will be the grouped
+  // `?x`, and the second column the variable `?y` of which we compute the
+  // average.
+  auto runTest = [this](bool inputIsLazy) {
+    std::vector<IdTable> tables;
+    tables.push_back(makeIdTableFromVector({{3, 6}, {8, 27}, {5, 7}}, I));
+    tables.push_back(makeIdTableFromVector({{8, 27}, {5, 9}}, I));
+    tables.push_back(makeIdTableFromVector({{5, 2}, {3, 4}}, I));
+    // The expected averages are as follows: (3 -> 5.0), (5 -> 6.0), (8
+    // -> 27.0).
+    auto subtree = ad_utility::makeExecutionTree<ValuesForTesting>(
+        qec, std::move(tables),
+        std::vector<std::optional<Variable>>{Variable{"?x"}, Variable{"?y"}});
+    auto& values =
+        dynamic_cast<ValuesForTesting&>(*subtree->getRootOperation());
+    values.forceFullyMaterialized() = !inputIsLazy;
+
+    SparqlExpressionPimpl avgYPimpl = makeAvgPimpl(varY);
+    std::vector<Alias> aliasesAvgY{Alias{avgYPimpl, Variable{"?avg"}}};
+
+    // Calculate result with optimization
+    qec->getQueryTreeCache().clearAll();
+    RuntimeParameters().set<"group-by-hash-map-enabled">(true);
+    GroupBy groupBy{qec, variablesOnlyX, aliasesAvgY, std::move(subtree)};
+    auto result = groupBy.computeResultOnlyForTesting();
+    ASSERT_TRUE(result.isFullyMaterialized());
+    EXPECT_THAT(
+        result.idTable(),
+        matchesIdTableFromVector({{I(3), D(5)}, {I(5), D(6)}, {I(8), D(27)}}));
+  };
+  runTest(true);
+  runTest(false);
+}
+
 // _____________________________________________________________________________
 TEST_F(GroupByOptimizations, correctResultForHashMapOptimizationForCountStar) {
   /* Setup query:
diff --git a/test/engine/ValuesForTesting.h b/test/engine/ValuesForTesting.h
index c02a9826bc..097ccd9c78 100644
--- a/test/engine/ValuesForTesting.h
+++ b/test/engine/ValuesForTesting.h
@@ -120,6 +120,8 @@ class ValuesForTesting : public Operation {
   }
   bool supportsLimit() const override { return supportsLimit_; }
 
+  bool& forceFullyMaterialized() { return forceFullyMaterialized_; }
+
  private:
   // ___________________________________________________________________________
   string getCacheKeyImpl() const override {