diff --git a/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/LeftOuterJoinToInnerJoinRule.java b/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/LeftOuterJoinToInnerJoinRule.java
new file mode 100644
index 0000000..d4a077e
--- /dev/null
+++ b/algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/LeftOuterJoinToInnerJoinRule.java
@@ -0,0 +1,110 @@
+/*
+ * Copyright 2009-2013 by The Regents of the University of California
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * you may obtain a copy of the License from
+ * 
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package edu.uci.ics.hyracks.algebricks.rewriter.rules;
+
+import org.apache.commons.lang3.mutable.Mutable;
+
+import edu.uci.ics.hyracks.algebricks.common.exceptions.AlgebricksException;
+import edu.uci.ics.hyracks.algebricks.common.utils.ListSet;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.ILogicalExpression;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.ILogicalOperator;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.IOptimizationContext;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalExpressionTag;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalOperatorTag;
+import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalVariable;
+import edu.uci.ics.hyracks.algebricks.core.algebra.expressions.ScalarFunctionCallExpression;
+import edu.uci.ics.hyracks.algebricks.core.algebra.expressions.VariableReferenceExpression;
+import edu.uci.ics.hyracks.algebricks.core.algebra.functions.AlgebricksBuiltinFunctions;
+import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.AbstractLogicalOperator;
+import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.InnerJoinOperator;
+import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.LeftOuterJoinOperator;
+import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.SelectOperator;
+import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.visitors.VariableUtilities;
+import edu.uci.ics.hyracks.algebricks.core.rewriter.base.IAlgebraicRewriteRule;
+
+/**
+ * This rule is to convert an outer join into an inner join when possible.
+ * 
+ * @author yingyib
+ */
+public class LeftOuterJoinToInnerJoinRule implements IAlgebraicRewriteRule {
+
+    @Override
+    public boolean rewritePre(Mutable<ILogicalOperator> opRef, IOptimizationContext context) throws AlgebricksException {
+        return false;
+    }
+
+    @Override
+    public boolean rewritePost(Mutable<ILogicalOperator> opRef, IOptimizationContext context)
+            throws AlgebricksException {
+        AbstractLogicalOperator op = (AbstractLogicalOperator) opRef.getValue();
+        if (op.getOperatorTag() != LogicalOperatorTag.SELECT) {
+            return false;
+        }
+        Mutable<ILogicalOperator> op2Ref = op.getInputs().get(0);
+        AbstractLogicalOperator op2 = (AbstractLogicalOperator) op2Ref.getValue();
+        if (op2.getOperatorTag() != LogicalOperatorTag.LEFTOUTERJOIN) {
+            return false;
+        }
+        SelectOperator selectOp = (SelectOperator) op;
+        LeftOuterJoinOperator joinOp = (LeftOuterJoinOperator) op2;
+        ILogicalExpression condition = selectOp.getCondition().getValue();
+        if (condition.getExpressionTag() != LogicalExpressionTag.FUNCTION_CALL) {
+            return false;
+        }
+        ScalarFunctionCallExpression func = (ScalarFunctionCallExpression) condition;
+        if (!convertable(func, joinOp)) {
+            return false;
+        }
+        ILogicalOperator newJoin = new InnerJoinOperator(joinOp.getCondition(), joinOp.getInputs().get(0), joinOp
+                .getInputs().get(1));
+        opRef.setValue(newJoin);
+        context.computeAndSetTypeEnvironmentForOperator(newJoin);
+        return true;
+    }
+
+    /**
+     * check if the condition is not(is-null(var)) and var is from the right branch of the join
+     */
+    private boolean convertable(ScalarFunctionCallExpression func, LeftOuterJoinOperator join)
+            throws AlgebricksException {
+        if (func.getFunctionIdentifier() != AlgebricksBuiltinFunctions.NOT) {
+            return false;
+        }
+        ILogicalExpression arg = func.getArguments().get(0).getValue();
+        if (arg.getExpressionTag() != LogicalExpressionTag.FUNCTION_CALL) {
+            return false;
+        }
+        ScalarFunctionCallExpression func2 = (ScalarFunctionCallExpression) arg;
+        if (func2.getFunctionIdentifier() != AlgebricksBuiltinFunctions.IS_NULL) {
+            return false;
+        }
+        ILogicalExpression arg2 = func2.getArguments().get(0).getValue();
+        if (arg2.getExpressionTag() != LogicalExpressionTag.VARIABLE) {
+            return false;
+        }
+        VariableReferenceExpression varExpr = (VariableReferenceExpression) arg2;
+        LogicalVariable var = varExpr.getVariableReference();
+        ListSet<LogicalVariable> leftVars = new ListSet<LogicalVariable>();
+        ListSet<LogicalVariable> rightVars = new ListSet<LogicalVariable>();
+        VariableUtilities.getLiveVariables(join.getInputs().get(0).getValue(), leftVars);
+        VariableUtilities.getLiveVariables(join.getInputs().get(1).getValue(), rightVars);
+        if (!rightVars.contains(var)) {
+            return false;
+        }
+        return true;
+    }
+
+}