diff --git a/processing/src/main/java/io/druid/query/topn/InvertedTopNMetricSpec.java b/processing/src/main/java/io/druid/query/topn/InvertedTopNMetricSpec.java
index e5a7fdfedde..ab5caa60a81 100644
--- a/processing/src/main/java/io/druid/query/topn/InvertedTopNMetricSpec.java
+++ b/processing/src/main/java/io/druid/query/topn/InvertedTopNMetricSpec.java
@@ -36,7 +36,6 @@ import java.util.List;
 public class InvertedTopNMetricSpec implements TopNMetricSpec
 {
   private static final byte CACHE_TYPE_ID = 0x3;
-
   private final TopNMetricSpec delegate;
 
   @JsonCreator
@@ -102,15 +101,27 @@ public class InvertedTopNMetricSpec implements TopNMetricSpec
     delegate.initTopNAlgorithmSelector(selector);
   }
 
+  @Override
+  public String getMetricName(DimensionSpec dimSpec)
+  {
+    return delegate.getMetricName(dimSpec);
+  }
+
   @Override
   public boolean equals(Object o)
   {
-    if (this == o) return true;
-    if (o == null || getClass() != o.getClass()) return false;
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
 
     InvertedTopNMetricSpec that = (InvertedTopNMetricSpec) o;
 
-    if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) return false;
+    if (delegate != null ? !delegate.equals(that.delegate) : that.delegate != null) {
+      return false;
+    }
 
     return true;
   }
diff --git a/processing/src/main/java/io/druid/query/topn/LexicographicTopNMetricSpec.java b/processing/src/main/java/io/druid/query/topn/LexicographicTopNMetricSpec.java
index 0f2c8344b14..5c1b9ab41e8 100644
--- a/processing/src/main/java/io/druid/query/topn/LexicographicTopNMetricSpec.java
+++ b/processing/src/main/java/io/druid/query/topn/LexicographicTopNMetricSpec.java
@@ -111,6 +111,12 @@ public class LexicographicTopNMetricSpec implements TopNMetricSpec
     selector.setAggregateAllMetrics(true);
   }
 
+  @Override
+  public String getMetricName(DimensionSpec dimSpec)
+  {
+    return dimSpec.getOutputName();
+  }
+
   @Override
   public String toString()
   {
diff --git a/processing/src/main/java/io/druid/query/topn/NumericTopNMetricSpec.java b/processing/src/main/java/io/druid/query/topn/NumericTopNMetricSpec.java
index 76f1a9341ac..fe5bce241a6 100644
--- a/processing/src/main/java/io/druid/query/topn/NumericTopNMetricSpec.java
+++ b/processing/src/main/java/io/druid/query/topn/NumericTopNMetricSpec.java
@@ -150,6 +150,12 @@ public class NumericTopNMetricSpec implements TopNMetricSpec
     selector.setAggregateTopNMetricFirst(true);
   }
 
+  @Override
+  public String getMetricName(DimensionSpec dimSpec)
+  {
+    return metric;
+  }
+
   @Override
   public String toString()
   {
diff --git a/processing/src/main/java/io/druid/query/topn/TopNBinaryFn.java b/processing/src/main/java/io/druid/query/topn/TopNBinaryFn.java
index 437c28f640f..f848f3c9e06 100644
--- a/processing/src/main/java/io/druid/query/topn/TopNBinaryFn.java
+++ b/processing/src/main/java/io/druid/query/topn/TopNBinaryFn.java
@@ -40,7 +40,7 @@ public class TopNBinaryFn implements BinaryFn<Result<TopNResultValue>, Result<To
   private final TopNResultMerger merger;
   private final DimensionSpec dimSpec;
   private final QueryGranularity gran;
-  private final String dimension;
+  private final DimensionSpec dimensionSpec;
   private final TopNMetricSpec topNMetricSpec;
   private final int threshold;
   private final List<AggregatorFactory> aggregations;
@@ -65,7 +65,7 @@ public class TopNBinaryFn implements BinaryFn<Result<TopNResultValue>, Result<To
     this.aggregations = aggregatorSpecs;
     this.postAggregations = postAggregatorSpecs;
 
-    this.dimension = dimSpec.getOutputName();
+    this.dimensionSpec = dimSpec;
     this.comparator = topNMetricSpec.getComparator(aggregatorSpecs, postAggregatorSpecs);
   }
 
@@ -79,11 +79,13 @@ public class TopNBinaryFn implements BinaryFn<Result<TopNResultValue>, Result<To
       return merger.getResult(arg1, comparator);
     }
 
-    Map<String, DimensionAndMetricValueExtractor> retVals = new LinkedHashMap<String, DimensionAndMetricValueExtractor>();
 
     TopNResultValue arg1Vals = arg1.getValue();
     TopNResultValue arg2Vals = arg2.getValue();
+    Map<String, DimensionAndMetricValueExtractor> retVals = new LinkedHashMap<String, DimensionAndMetricValueExtractor>();
 
+    String dimension = dimensionSpec.getOutputName();
+    String topNMetricName = topNMetricSpec.getMetricName(dimensionSpec);
     for (DimensionAndMetricValueExtractor arg1Val : arg1Vals) {
       retVals.put(arg1Val.getStringDimensionValue(dimension), arg1Val);
     }
@@ -92,16 +94,17 @@ public class TopNBinaryFn implements BinaryFn<Result<TopNResultValue>, Result<To
       DimensionAndMetricValueExtractor arg1Val = retVals.get(dimensionValue);
 
       if (arg1Val != null) {
-        Map<String, Object> retVal = new LinkedHashMap<String, Object>();
+        Map<String, Object> retVal = new LinkedHashMap<String, Object>(aggregations.size() + 2);
 
         retVal.put(dimension, dimensionValue);
         for (AggregatorFactory factory : aggregations) {
           final String metricName = factory.getName();
           retVal.put(metricName, factory.combine(arg1Val.getMetric(metricName), arg2Val.getMetric(metricName)));
         }
-
-        for (PostAggregator pf : postAggregations) {
-          retVal.put(pf.getName(), pf.compute(retVal));
+        for (PostAggregator postAgg : postAggregations) {
+          if (postAgg.getName().equals(topNMetricName)) {
+            retVal.put(postAgg.getName(), postAgg.compute(retVal));
+          }
         }
 
         retVals.put(dimensionValue, new DimensionAndMetricValueExtractor(retVal));
diff --git a/processing/src/main/java/io/druid/query/topn/TopNLexicographicResultBuilder.java b/processing/src/main/java/io/druid/query/topn/TopNLexicographicResultBuilder.java
index 37360dfb1cd..b12f4c85b51 100644
--- a/processing/src/main/java/io/druid/query/topn/TopNLexicographicResultBuilder.java
+++ b/processing/src/main/java/io/druid/query/topn/TopNLexicographicResultBuilder.java
@@ -75,9 +75,6 @@ public class TopNLexicographicResultBuilder implements TopNResultBuilder
       for (Object metricVal : metricVals) {
         metricValues.put(aggsIter.next().getName(), metricVal);
       }
-      for (PostAggregator postAgg : postAggs) {
-        metricValues.put(postAgg.getName(), postAgg.compute(metricValues));
-      }
 
       pQueue.add(new DimValHolder.Builder().withDirName(dimName).withMetricValues(metricValues).build());
     }
diff --git a/processing/src/main/java/io/druid/query/topn/TopNMetricSpec.java b/processing/src/main/java/io/druid/query/topn/TopNMetricSpec.java
index c2baf13e3eb..44103f93708 100644
--- a/processing/src/main/java/io/druid/query/topn/TopNMetricSpec.java
+++ b/processing/src/main/java/io/druid/query/topn/TopNMetricSpec.java
@@ -55,4 +55,6 @@ public interface TopNMetricSpec
   public <T> TopNMetricSpecBuilder<T> configureOptimizer(TopNMetricSpecBuilder<T> builder);
 
   public void initTopNAlgorithmSelector(TopNAlgorithmSelector selector);
+
+  public String getMetricName(DimensionSpec dimSpec);
 }
diff --git a/processing/src/main/java/io/druid/query/topn/TopNNumericResultBuilder.java b/processing/src/main/java/io/druid/query/topn/TopNNumericResultBuilder.java
index 9f6479baee4..a806c7966ec 100644
--- a/processing/src/main/java/io/druid/query/topn/TopNNumericResultBuilder.java
+++ b/processing/src/main/java/io/druid/query/topn/TopNNumericResultBuilder.java
@@ -40,7 +40,6 @@ public class TopNNumericResultBuilder implements TopNResultBuilder
   private final DateTime timestamp;
   private final DimensionSpec dimSpec;
   private final String metricName;
-
   private MinMaxPriorityQueue<DimValHolder> pQueue = null;
 
   public TopNNumericResultBuilder(
@@ -75,8 +74,12 @@ public class TopNNumericResultBuilder implements TopNResultBuilder
     for (Object metricVal : metricVals) {
       metricValues.put(aggFactoryIter.next().getName(), metricVal);
     }
+
     for (PostAggregator postAgg : postAggs) {
-      metricValues.put(postAgg.getName(), postAgg.compute(metricValues));
+      if (postAgg.getName().equals(metricName)) {
+        metricValues.put(postAgg.getName(), postAgg.compute(metricValues));
+        break;
+      }
     }
 
     Object topNMetricVal = metricValues.get(metricName);
diff --git a/processing/src/main/java/io/druid/query/topn/TopNQueryQueryToolChest.java b/processing/src/main/java/io/druid/query/topn/TopNQueryQueryToolChest.java
index a7d77fde396..0c78f6305fe 100644
--- a/processing/src/main/java/io/druid/query/topn/TopNQueryQueryToolChest.java
+++ b/processing/src/main/java/io/druid/query/topn/TopNQueryQueryToolChest.java
@@ -161,7 +161,12 @@ public class TopNQueryQueryToolChest extends QueryToolChest<Result<TopNResultVal
                       values.put(agg.getName(), fn.manipulate(agg, input.getMetric(agg.getName())));
                     }
                     for (PostAggregator postAgg : query.getPostAggregatorSpecs()) {
-                      values.put(postAgg.getName(), input.getMetric(postAgg.getName()));
+                      Object calculatedPostAgg = input.getMetric(postAgg.getName());
+                      if (calculatedPostAgg != null) {
+                        values.put(postAgg.getName(), input.getMetric(postAgg.getName()));
+                      } else {
+                        values.put(postAgg.getName(), postAgg.compute(values));
+                      }
                     }
                     values.put(dimension, input.getDimensionValue(dimension));
 
@@ -281,10 +286,6 @@ public class TopNQueryQueryToolChest extends QueryToolChest<Result<TopNResultVal
                 vals.put(factory.getName(), factory.deserialize(resultIter.next()));
               }
 
-              for (PostAggregator postAgg : postAggs) {
-                vals.put(postAgg.getName(), postAgg.compute(vals));
-              }
-
               retVal.add(vals);
             }
 
diff --git a/processing/src/test/java/io/druid/query/topn/TopNBinaryFnBenchmark.java b/processing/src/test/java/io/druid/query/topn/TopNBinaryFnBenchmark.java
new file mode 100644
index 00000000000..65ab16ad19b
--- /dev/null
+++ b/processing/src/test/java/io/druid/query/topn/TopNBinaryFnBenchmark.java
@@ -0,0 +1,135 @@
+/*
+ * Druid - a distributed column store.
+ * Copyright (C) 2012, 2013, 2014  Metamarkets Group Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+
+package io.druid.query.topn;
+
+import com.google.caliper.Param;
+import com.google.caliper.Runner;
+import com.google.caliper.SimpleBenchmark;
+import com.google.common.collect.Lists;
+import io.druid.granularity.QueryGranularity;
+import io.druid.query.Result;
+import io.druid.query.aggregation.AggregatorFactory;
+import io.druid.query.aggregation.CountAggregatorFactory;
+import io.druid.query.aggregation.LongSumAggregatorFactory;
+import io.druid.query.aggregation.PostAggregator;
+import io.druid.query.aggregation.post.ArithmeticPostAggregator;
+import io.druid.query.aggregation.post.ConstantPostAggregator;
+import io.druid.query.aggregation.post.FieldAccessPostAggregator;
+import io.druid.query.dimension.DefaultDimensionSpec;
+import org.joda.time.DateTime;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class TopNBinaryFnBenchmark extends SimpleBenchmark
+{
+  @Param({"1", "5", "10", "15"})
+  int aggCount;
+  @Param({"1", "5", "10", "15"})
+  int postAggCount;
+  @Param({"1000", "10000"})
+  int threshold;
+  Result<TopNResultValue> result1;
+  Result<TopNResultValue> result2;
+  TopNBinaryFn fn;
+
+  public static void main(String[] args) throws Exception
+  {
+    Runner.main(TopNBinaryFnBenchmark.class, args);
+  }
+
+  @Override
+  protected void setUp() throws Exception
+  {
+
+    final ConstantPostAggregator constant = new ConstantPostAggregator("const", 1L, null);
+    final FieldAccessPostAggregator rowsPostAgg = new FieldAccessPostAggregator("rows", "rows");
+    final FieldAccessPostAggregator indexPostAgg = new FieldAccessPostAggregator("index", "index");
+
+
+    final List<AggregatorFactory> aggregatorFactories = new ArrayList<>();
+    aggregatorFactories.add(new CountAggregatorFactory("rows"));
+    aggregatorFactories.add(new LongSumAggregatorFactory("index", "index"));
+    for (int i = 1; i < aggCount; i++) {
+      aggregatorFactories.add(new CountAggregatorFactory("rows" + i));
+    }
+    final List<PostAggregator> postAggregators = new ArrayList<>();
+    for (int i = 0; i < postAggCount; i++) {
+      postAggregators.add(
+          new ArithmeticPostAggregator(
+              "addrowsindexconstant" + i,
+              "+",
+              Lists.newArrayList(constant, rowsPostAgg, indexPostAgg)
+          )
+      );
+    }
+    final DateTime currTime = new DateTime();
+    List<Map<String, Object>> list = new ArrayList<>();
+    for (int i = 0; i < threshold; i++) {
+      Map<String, Object> res = new HashMap<>();
+      res.put("testdim", "" + i);
+      res.put("rows", 1L);
+      for (int j = 0; j < aggCount; j++) {
+        res.put("rows" + j, 1L);
+      }
+      res.put("index", 1L);
+      list.add(res);
+    }
+    result1 = new Result<>(
+        currTime,
+        new TopNResultValue(list)
+    );
+
+    List<Map<String, Object>> list2 = new ArrayList<>();
+    for (int i = 0; i < threshold; i++) {
+      Map<String, Object> res = new HashMap<>();
+      res.put("testdim", "" + i);
+      res.put("rows", 2L);
+      for (int j = 0; j < aggCount; j++) {
+        res.put("rows" + j, 2L);
+      }
+      res.put("index", 2L);
+      list2.add(res);
+    }
+    result2 = new Result<>(
+        currTime,
+        new TopNResultValue(list2)
+    );
+    fn = new TopNBinaryFn(
+        TopNResultMerger.identity,
+        QueryGranularity.ALL,
+        new DefaultDimensionSpec("testdim", null),
+        new NumericTopNMetricSpec("index"),
+        100,
+        aggregatorFactories,
+        postAggregators
+    );
+  }
+
+  public void timeMerge(int nReps)
+  {
+    for (int i = 0; i < nReps; i++) {
+      fn.apply(result1, result2);
+    }
+  }
+
+}
diff --git a/processing/src/test/java/io/druid/query/topn/TopNBinaryFnTest.java b/processing/src/test/java/io/druid/query/topn/TopNBinaryFnTest.java
index 0eba9778c4e..cb3089d6397 100644
--- a/processing/src/test/java/io/druid/query/topn/TopNBinaryFnTest.java
+++ b/processing/src/test/java/io/druid/query/topn/TopNBinaryFnTest.java
@@ -129,15 +129,13 @@ public class TopNBinaryFnTest
                 ImmutableMap.<String, Object>of(
                     "testdim", "1",
                     "rows", 3L,
-                    "index", 5L,
-                    "addrowsindexconstant", 9.0
+                    "index", 5L
                 ),
 
                 ImmutableMap.<String, Object>of(
                     "testdim", "2",
                     "rows", 4L,
-                    "index", 4L,
-                    "addrowsindexconstant", 9.0
+                    "index", 4L
                 )
             )
         )
@@ -214,14 +212,12 @@ public class TopNBinaryFnTest
                 ImmutableMap.<String, Object>of(
                     "testdim", "1",
                     "rows", 3L,
-                    "index", 5L,
-                    "addrowsindexconstant", 9.0
+                    "index", 5L
                 ),
                 ImmutableMap.<String, Object>of(
                     "testdim", "2",
                     "rows", 4L,
-                    "index", 4L,
-                    "addrowsindexconstant", 9.0
+                    "index", 4L
                 )
             )
         )
@@ -427,15 +423,12 @@ public class TopNBinaryFnTest
                 ImmutableMap.<String, Object>of(
                     "testdim", "1",
                     "rows", 3L,
-                    "index", 5L,
-                    "addrowsindexconstant", 9.0
+                    "index", 5L
                 ),
                 ImmutableMap.<String, Object>of(
                     "testdim", "2",
                     "rows", 4L,
-                    "index", 4L,
-                    "addrowsindexconstant", 9.0
-                )
+                    "index", 4L                )
             )
         )
     );