diff --git a/processing/src/main/java/io/druid/query/GroupByParallelQueryRunner.java b/processing/src/main/java/io/druid/query/GroupByParallelQueryRunner.java
index 31938f8631b..552bea9085b 100644
--- a/processing/src/main/java/io/druid/query/GroupByParallelQueryRunner.java
+++ b/processing/src/main/java/io/druid/query/GroupByParallelQueryRunner.java
@@ -44,6 +44,7 @@ import io.druid.segment.incremental.IncrementalIndex;
 import java.nio.ByteBuffer;
 import java.util.List;
 import java.util.Map;
+import java.util.Queue;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
@@ -84,7 +85,7 @@ public class GroupByParallelQueryRunner<T> implements QueryRunner<T>
         configSupplier.get(),
         bufferPool
     );
-    final Pair<List, Accumulator<List, T>> bySegmentAccumulatorPair = GroupByQueryHelper.createBySegmentAccumulatorPair();
+    final Pair<Queue, Accumulator<Queue, T>> bySegmentAccumulatorPair = GroupByQueryHelper.createBySegmentAccumulatorPair();
     final boolean bySegment = query.getContextBySegment(false);
     final int priority = query.getContextPriority(0);
 
diff --git a/processing/src/main/java/io/druid/query/groupby/GroupByQueryHelper.java b/processing/src/main/java/io/druid/query/groupby/GroupByQueryHelper.java
index 597e3ba821f..5f07b6fcf39 100644
--- a/processing/src/main/java/io/druid/query/groupby/GroupByQueryHelper.java
+++ b/processing/src/main/java/io/druid/query/groupby/GroupByQueryHelper.java
@@ -36,7 +36,10 @@ import io.druid.segment.incremental.OffheapIncrementalIndex;
 import io.druid.segment.incremental.OnheapIncrementalIndex;
 
 import java.nio.ByteBuffer;
+import java.util.Collections;
 import java.util.List;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
 
 public class GroupByQueryHelper
 {
@@ -130,18 +133,19 @@ public class GroupByQueryHelper
     return new Pair<>(index, accumulator);
   }
 
-  public static <T> Pair<List, Accumulator<List, T>> createBySegmentAccumulatorPair()
+  public static <T> Pair<Queue, Accumulator<Queue, T>> createBySegmentAccumulatorPair()
   {
-    List init = Lists.newArrayList();
-    Accumulator<List, T> accumulator = new Accumulator<List, T>()
+    // In parallel query runner multiple threads add to this queue concurrently
+    Queue init = new ConcurrentLinkedQueue<>();
+    Accumulator<Queue, T> accumulator = new Accumulator<Queue, T>()
     {
       @Override
-      public List accumulate(List accumulated, T in)
+      public Queue accumulate(Queue accumulated, T in)
       {
         if(in == null){
           throw new ISE("Cannot have null result");
         }
-        accumulated.add(in);
+        accumulated.offer(in);
         return accumulated;
       }
     };
diff --git a/processing/src/main/java/io/druid/query/groupby/GroupByQueryRunnerFactory.java b/processing/src/main/java/io/druid/query/groupby/GroupByQueryRunnerFactory.java
index 9ac86ffc523..58ccf98c083 100644
--- a/processing/src/main/java/io/druid/query/groupby/GroupByQueryRunnerFactory.java
+++ b/processing/src/main/java/io/druid/query/groupby/GroupByQueryRunnerFactory.java
@@ -49,6 +49,7 @@ import io.druid.segment.incremental.IncrementalIndex;
 import java.nio.ByteBuffer;
 import java.util.List;
 import java.util.Map;
+import java.util.Queue;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
@@ -115,7 +116,7 @@ public class GroupByQueryRunnerFactory implements QueryRunnerFactory<Row, GroupB
                               config.get(),
                               computationBufferPool
                           );
-                      final Pair<List, Accumulator<List, Row>> bySegmentAccumulatorPair = GroupByQueryHelper.createBySegmentAccumulatorPair();
+                      final Pair<Queue, Accumulator<Queue, Row>> bySegmentAccumulatorPair = GroupByQueryHelper.createBySegmentAccumulatorPair();
                       final int priority = query.getContextPriority(0);
                       final boolean bySegment = query.getContextBySegment(false);
 
diff --git a/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerTest.java b/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerTest.java
index 89dec7c2e71..da31afa0dde 100644
--- a/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerTest.java
+++ b/processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerTest.java
@@ -22,6 +22,7 @@ import com.google.common.base.Function;
 import com.google.common.base.Supplier;
 import com.google.common.base.Suppliers;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
@@ -33,9 +34,14 @@ import io.druid.data.input.Row;
 import io.druid.granularity.PeriodGranularity;
 import io.druid.granularity.QueryGranularity;
 import io.druid.jackson.DefaultObjectMapper;
+import io.druid.query.BySegmentResultValue;
+import io.druid.query.BySegmentResultValueClass;
+import io.druid.query.FinalizeResultsQueryRunner;
 import io.druid.query.Query;
 import io.druid.query.QueryRunner;
 import io.druid.query.QueryRunnerTestHelper;
+import io.druid.query.QueryToolChest;
+import io.druid.query.Result;
 import io.druid.query.TestQueryRunners;
 import io.druid.query.aggregation.AggregatorFactory;
 import io.druid.query.aggregation.DoubleSumAggregatorFactory;
@@ -88,6 +94,8 @@ import java.util.Collection;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 
 @RunWith(Parameterized.class)
 public class GroupByQueryRunnerTest
@@ -2337,4 +2345,59 @@ public class GroupByQueryRunnerTest
     Iterable<Row> results = GroupByQueryRunnerTestHelper.runQuery(factory, runner, query);
     TestHelper.assertExpectedObjects(expectedResults, results, "");
   }
+
+  @Test
+  public void testBySegmentResults()
+  {
+    int segmentCount = 32;
+    Result<BySegmentResultValue> singleSegmentResult = new Result<BySegmentResultValue>(
+        new DateTime("2011-01-12T00:00:00.000Z"),
+        new BySegmentResultValueClass(
+            Arrays.asList(
+                GroupByQueryRunnerTestHelper.createExpectedRow(
+                    "2011-04-01",
+                    "alias",
+                    "mezzanine",
+                    "rows",
+                    6L,
+                    "idx",
+                    4420L
+                )
+            ), "testSegment", new Interval("2011-04-02T00:00:00.000Z/2011-04-04T00:00:00.000Z")
+        )
+    );
+    List<Result> bySegmentResults = Lists.newArrayList();
+    for (int i = 0; i < segmentCount; i++) {
+      bySegmentResults.add(singleSegmentResult);
+    }
+    GroupByQuery.Builder builder = GroupByQuery
+        .builder()
+        .setDataSource(QueryRunnerTestHelper.dataSource)
+        .setInterval("2011-04-02/2011-04-04")
+        .setDimensions(Lists.<DimensionSpec>newArrayList(new DefaultDimensionSpec("quality", "alias")))
+        .setAggregatorSpecs(
+            Arrays.asList(
+                QueryRunnerTestHelper.rowsCount,
+                new LongSumAggregatorFactory("idx", "index")
+            )
+        )
+        .setGranularity(new PeriodGranularity(new Period("P1M"), null, null))
+        .setDimFilter(new SelectorDimFilter("quality", "mezzanine"))
+        .setContext(ImmutableMap.<String, Object>of("bySegment", true));
+    final GroupByQuery fullQuery = builder.build();
+    QueryToolChest toolChest = factory.getToolchest();
+
+    List<QueryRunner<Row>> singleSegmentRunners = Lists.newArrayList();
+    for (int i = 0; i < segmentCount; i++) {
+      singleSegmentRunners.add(toolChest.preMergeQueryDecoration(runner));
+    }
+    ExecutorService exec = Executors.newCachedThreadPool();
+    QueryRunner theRunner = new FinalizeResultsQueryRunner<>(
+        toolChest.mergeResults(factory.mergeRunners(Executors.newCachedThreadPool(), singleSegmentRunners)),
+        toolChest
+    );
+
+    TestHelper.assertExpectedObjects(bySegmentResults, theRunner.run(fullQuery, Maps.newHashMap()), "");
+    exec.shutdownNow();
+  }
 }