diff --git a/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/IntroduceGroupByForSubplanRule.java b/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/IntroduceGroupByForSubplanRule.java
index 18c483e..489167f 100644
--- a/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/IntroduceGroupByForSubplanRule.java
+++ b/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/IntroduceGroupByForSubplanRule.java
@@ -143,8 +143,10 @@
 
         Set<LogicalVariable> pkVars = computeGbyVars(outerNts, free, context);
         if (pkVars == null || pkVars.size() < 1) {
-            // could not group only by primary keys
-            return false;
+            // there is no non-trivial primary key, group-by keys are all live variables
+            ILogicalOperator subplanInput = subplan.getInputs().get(0).getValue();
+            pkVars = new HashSet<LogicalVariable>();
+            VariableUtilities.getLiveVariables(subplanInput, pkVars);
         }
         AlgebricksConfig.ALGEBRICKS_LOGGER.fine("Found FD for introducing group-by: " + pkVars);