diff --git a/processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java b/processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java
index fa7bbc0b86e..8482fdd9e73 100644
--- a/processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java
+++ b/processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java
@@ -274,7 +274,10 @@ public class GroupByQueryQueryToolChest extends QueryToolChest<Row, GroupByQuery
                       "finalize", false,
                       //setting sort to false avoids unnecessary sorting while merging results. we only need to sort
                       //in the end when returning results to user.
-                      GroupByQueryHelper.CTX_KEY_SORT_RESULTS, false
+                      GroupByQueryHelper.CTX_KEY_SORT_RESULTS, false,
+                      //no merging needed at historicals because GroupByQueryRunnerFactory.mergeRunners(..) would return
+                      //merged results
+                      GROUP_BY_MERGE_KEY, false
                   )
               )
               , context
diff --git a/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerFactoryTest.java b/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerFactoryTest.java
new file mode 100644
index 00000000000..bc1b26f8b14
--- /dev/null
+++ b/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerFactoryTest.java
@@ -0,0 +1,188 @@
+/*
+ *
+ *  Licensed to Metamarkets Group Inc. (Metamarkets) under one
+ *  or more contributor license agreements. See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership. Metamarkets licenses this file
+ *  to you 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 at
+ *
+ *  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 io.druid.query.groupby;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.base.Supplier;
+import com.google.common.base.Suppliers;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Lists;
+import com.google.common.collect.Maps;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.metamx.common.guava.Sequence;
+import com.metamx.common.guava.Sequences;
+import io.druid.collections.StupidPool;
+import io.druid.data.input.Row;
+import io.druid.data.input.impl.CSVParseSpec;
+import io.druid.data.input.impl.DimensionsSpec;
+import io.druid.data.input.impl.StringInputRowParser;
+import io.druid.data.input.impl.TimestampSpec;
+import io.druid.granularity.QueryGranularity;
+import io.druid.jackson.DefaultObjectMapper;
+import io.druid.query.Query;
+import io.druid.query.QueryRunner;
+import io.druid.query.QueryRunnerFactory;
+import io.druid.query.QueryRunnerTestHelper;
+import io.druid.query.QueryWatcher;
+import io.druid.query.aggregation.AggregatorFactory;
+import io.druid.query.aggregation.CountAggregatorFactory;
+import io.druid.query.dimension.DefaultDimensionSpec;
+import io.druid.query.dimension.DimensionSpec;
+import io.druid.query.spec.LegacySegmentSpec;
+import io.druid.segment.CloserRule;
+import io.druid.segment.IncrementalIndexSegment;
+import io.druid.segment.Segment;
+import io.druid.segment.TestHelper;
+import io.druid.segment.incremental.IncrementalIndex;
+import io.druid.segment.incremental.OnheapIncrementalIndex;
+import org.junit.Rule;
+import org.junit.Test;
+
+import java.nio.ByteBuffer;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.concurrent.Executors;
+
+/**
+ */
+public class GroupByQueryRunnerFactoryTest
+{
+  @Rule
+  public CloserRule closerRule = new CloserRule(true);
+
+  @Test
+  public void testMergeRunnersEnsureGroupMerging() throws Exception
+  {
+    QueryRunnerFactory factory = createFactory();
+    QueryRunner mergedRunner = factory.mergeRunners(
+        Executors.newSingleThreadExecutor(),
+        ImmutableList.of(
+        factory.createRunner(createSegment()),
+        factory.createRunner(createSegment())
+        )
+    );
+
+    GroupByQuery query = GroupByQuery
+        .builder()
+        .setDataSource("xx")
+        .setQuerySegmentSpec(new LegacySegmentSpec("1970/3000"))
+        .setGranularity(QueryGranularity.ALL)
+        .setDimensions(Lists.<DimensionSpec>newArrayList(new DefaultDimensionSpec("tags", "tags")))
+        .setAggregatorSpecs(
+            Arrays.asList(
+                new AggregatorFactory[]
+                    {
+                        new CountAggregatorFactory("count")
+                    }
+            )
+        )
+        .build();
+
+    Sequence<Row> result = mergedRunner.run(query, Maps.newHashMap());
+
+    List<Row> expectedResults = Arrays.asList(
+        GroupByQueryRunnerTestHelper.createExpectedRow("1970-01-01T00:00:00.000Z", "tags", "t1", "count", 2L),
+        GroupByQueryRunnerTestHelper.createExpectedRow("1970-01-01T00:00:00.000Z", "tags", "t2", "count", 4L)
+    );
+
+    TestHelper.assertExpectedObjects(expectedResults, Sequences.toList(result, new ArrayList<Row>()), "");
+  }
+
+  private Segment createSegment() throws Exception
+  {
+    IncrementalIndex incrementalIndex = new OnheapIncrementalIndex(
+        0,
+        QueryGranularity.NONE,
+        new AggregatorFactory[]{
+            new CountAggregatorFactory("count")
+        },
+        true,
+        true,
+        true,
+        5000
+    );
+
+    StringInputRowParser parser = new StringInputRowParser(
+        new CSVParseSpec(
+            new TimestampSpec("timestamp", "iso", null),
+            new DimensionsSpec(DimensionsSpec.getDefaultSchemas(ImmutableList.of("product", "tags")), null, null),
+            "\t",
+            ImmutableList.of("timestamp", "product", "tags")
+        ),
+        "UTF-8"
+    );
+
+    String[] rows = new String[]{
+        "2011-01-12T00:00:00.000Z,product_1,t1",
+        "2011-01-13T00:00:00.000Z,product_2,t2",
+        "2011-01-14T00:00:00.000Z,product_3,t2",
+    };
+
+    for (String row : rows) {
+      incrementalIndex.add(parser.parse(row));
+    }
+
+    closerRule.closeLater(incrementalIndex);
+
+    return new IncrementalIndexSegment(incrementalIndex, "test");
+  }
+
+  private GroupByQueryRunnerFactory createFactory()
+  {
+    ObjectMapper mapper = new DefaultObjectMapper();
+
+    Supplier<GroupByQueryConfig> configSupplier = Suppliers.ofInstance(new GroupByQueryConfig());
+    StupidPool<ByteBuffer> pool = new StupidPool<>(
+        new Supplier<ByteBuffer>()
+        {
+          @Override
+          public ByteBuffer get()
+          {
+            return ByteBuffer.allocate(1024 * 1024);
+          }
+        });
+
+    QueryWatcher noopQueryWatcher = new QueryWatcher()
+    {
+      @Override
+      public void registerQuery(Query query, ListenableFuture future)
+      {
+
+      }
+    };
+
+    GroupByQueryEngine engine = new GroupByQueryEngine(configSupplier, pool);
+    GroupByQueryQueryToolChest toolchest = new GroupByQueryQueryToolChest(
+        configSupplier, mapper, engine, pool,
+        QueryRunnerTestHelper.NoopIntervalChunkingQueryRunnerDecorator()
+    );
+    return new GroupByQueryRunnerFactory(
+        engine,
+        noopQueryWatcher,
+        configSupplier,
+        toolchest,
+        pool
+    );
+  }
+}