diff --git a/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerSqlV2Test.java b/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerSqlV2Test.java
index db3d517f0b..8538ebbae6 100644
--- a/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerSqlV2Test.java
+++ b/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerSqlV2Test.java
@@ -506,4 +506,45 @@ SELECT name, ROW_NUMBER() OVER (ORDER BY id) AS rn FROM catalog.employees
               LogicalTableScan(table=[[catalog, employees]])
             """);
   }
+
+  @Test
+  public void testGroupByExpression() {
+    givenQuery("SELECT LENGTH(name), COUNT(*) FROM catalog.employees GROUP BY LENGTH(name)")
+        .assertPlan(
+            """
+            LogicalAggregate(group=[{0}], COUNT(*)=[COUNT()])
+              LogicalProject(LENGTH(name)=[CHAR_LENGTH($1)])
+                LogicalTableScan(table=[[catalog, employees]])
+            """);
+  }
+
+  @Test
+  public void testHavingOnGroupByExpression() {
+    givenQuery(
+            "SELECT COUNT(*) FROM catalog.employees GROUP BY LENGTH(name) HAVING LENGTH(name) > 3")
+        .assertPlan(
+            """
+            LogicalProject(COUNT(*)=[$0])
+              LogicalFilter(condition=[>($1, 3)])
+                LogicalProject(COUNT(*)=[$1], LENGTH(name)=[$0])
+                  LogicalAggregate(group=[{0}], COUNT(*)=[COUNT()])
+                    LogicalProject(LENGTH(name)=[CHAR_LENGTH($1)])
+                      LogicalTableScan(table=[[catalog, employees]])
+            """);
+  }
+
+  @Test
+  public void testOrderByGroupByExpression() {
+    givenQuery(
+            """
+            SELECT LENGTH(name) FROM catalog.employees GROUP BY LENGTH(name) ORDER BY LENGTH(name)
+            """)
+        .assertPlan(
+            """
+            LogicalSort(sort0=[$0], dir0=[ASC-nulls-first])
+              LogicalAggregate(group=[{0}])
+                LogicalProject(LENGTH(name)=[CHAR_LENGTH($1)])
+                  LogicalTableScan(table=[[catalog, employees]])
+            """);
+  }
 }
diff --git a/core/src/main/java/org/opensearch/sql/calcite/CalcitePlanContext.java b/core/src/main/java/org/opensearch/sql/calcite/CalcitePlanContext.java
index f18ac0b047..7ca7ab0930 100644
--- a/core/src/main/java/org/opensearch/sql/calcite/CalcitePlanContext.java
+++ b/core/src/main/java/org/opensearch/sql/calcite/CalcitePlanContext.java
@@ -22,6 +22,7 @@
 import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.tools.FrameworkConfig;
 import org.opensearch.sql.ast.expression.AggregateFunction;
+import org.opensearch.sql.ast.expression.Function;
 import org.opensearch.sql.ast.expression.UnresolvedExpression;
 import org.opensearch.sql.ast.tree.HighlightConfig;
 import org.opensearch.sql.calcite.utils.CalciteToolsHelper;
@@ -71,6 +72,9 @@ public class CalcitePlanContext {
    */
   @Getter private final Map<AggregateFunction, Integer> aggregateOutputIndex = new HashMap<>();
 
+  /** Maps GROUP BY Function AST nodes to their output field index for post-aggregate resolution. */
+  @Getter private final Map<Function, Integer> groupKeyOutputIndex = new HashMap<>();
+
   /**
    * List of captured variables from outer scope for lambda functions. When a lambda body references
    * a field that is not a lambda parameter, it gets captured and stored here. The captured
diff --git a/core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java b/core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java
index 2ed139d781..2658066bfb 100644
--- a/core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java
+++ b/core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java
@@ -1788,6 +1788,14 @@ private void visitAggregation(
         context.getAggregateOutputIndex().put(aggFunc, aggStartIdx + i);
       }
     }
+    context.getGroupKeyOutputIndex().clear();
+    int groupStartIdx = metricsFirst ? aggRexList.size() : 0;
+    for (int i = 0; i < groupExprList.size(); i++) {
+      Function groupFunc = extractFunction(groupExprList.get(i));
+      if (groupFunc != null) {
+        context.getGroupKeyOutputIndex().put(groupFunc, groupStartIdx + i);
+      }
+    }
   }
 
   private static AggregateFunction extractAggregateFunction(UnresolvedExpression expr) {
@@ -1796,6 +1804,12 @@ private static AggregateFunction extractAggregateFunction(UnresolvedExpression e
     return null;
   }
 
+  private static Function extractFunction(UnresolvedExpression expr) {
+    if (expr instanceof Function f) return f;
+    if (expr instanceof Alias alias) return extractFunction(alias.getDelegated());
+    return null;
+  }
+
   /**
    * Collects input refs used by aggregate FILTER(WHERE ...) predicates so trimming retains them.
    */
diff --git a/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java b/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
index 1d473b168e..0c1326f868 100644
--- a/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
+++ b/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
@@ -563,6 +563,12 @@ private List<RelDataType> modifyLambdaTypeByFunction(
 
   @Override
   public RexNode visitFunction(Function node, CalcitePlanContext context) {
+    // Post-aggregate, a GROUP BY function expression is a materialized output column; reference it
+    // instead of recomputing from base fields the aggregation removed.
+    Integer groupKeyIndex = context.getGroupKeyOutputIndex().get(node);
+    if (groupKeyIndex != null) {
+      return context.relBuilder.field(groupKeyIndex);
+    }
     List<UnresolvedExpression> args = node.getFuncArgs();
     List<RexNode> arguments = new ArrayList<>();