diff --git a/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/operators/logical/visitors/UsedVariableVisitor.java b/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/operators/logical/visitors/UsedVariableVisitor.java
index 2feea5d..9f62124 100644
--- a/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/operators/logical/visitors/UsedVariableVisitor.java
+++ b/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/operators/logical/visitors/UsedVariableVisitor.java
@@ -399,6 +399,9 @@
         for (Mutable<ILogicalExpression> e : op.getSecondaryKeyExpressions()) {
             e.getValue().getUsedVariables(usedVariables);
         }
+        if (op.getFilterExpression() != null) {
+            op.getFilterExpression().getValue().getUsedVariables(usedVariables);
+        }
         if (op.getAdditionalFilteringExpressions() != null) {
             for (Mutable<ILogicalExpression> e : op.getAdditionalFilteringExpressions()) {
                 e.getValue().getUsedVariables(usedVariables);