diff --git a/indexer/src/main/java/com/metamx/druid/indexer/DeterminePartitionsJob.java b/indexer/src/main/java/com/metamx/druid/indexer/DeterminePartitionsJob.java
index 425b33cedff..a0165b47517 100644
--- a/indexer/src/main/java/com/metamx/druid/indexer/DeterminePartitionsJob.java
+++ b/indexer/src/main/java/com/metamx/druid/indexer/DeterminePartitionsJob.java
@@ -28,6 +28,7 @@ import com.google.common.base.Splitter;
 import com.google.common.base.Throwables;
 import com.google.common.collect.ComparisonChain;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSortedSet;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Iterators;
@@ -55,6 +56,7 @@ import org.apache.hadoop.mapred.InvalidJobConfException;
 import org.apache.hadoop.mapreduce.Job;
 import org.apache.hadoop.mapreduce.JobContext;
 import org.apache.hadoop.mapreduce.Mapper;
+import org.apache.hadoop.mapreduce.Partitioner;
 import org.apache.hadoop.mapreduce.RecordWriter;
 import org.apache.hadoop.mapreduce.Reducer;
 import org.apache.hadoop.mapreduce.TaskAttemptContext;
@@ -71,6 +73,7 @@ import org.joda.time.Interval;
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.nio.ByteBuffer;
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
@@ -189,6 +192,8 @@ public class DeterminePartitionsJob implements Jobby
       dimSelectionJob.setOutputValueClass(Text.class);
       dimSelectionJob.setOutputFormatClass(DeterminePartitionsDimSelectionOutputFormat.class);
       dimSelectionJob.setJarByClass(DeterminePartitionsJob.class);
+      dimSelectionJob.setPartitionerClass(DeterminePartitionsDimSelectionPartitioner.class);
+      dimSelectionJob.setNumReduceTasks(config.getGranularitySpec().bucketIntervals().size());
 
       config.intoConfiguration(dimSelectionJob);
       FileOutputFormat.setOutputPath(dimSelectionJob, config.makeIntermediatePath());
@@ -371,11 +376,21 @@ public class DeterminePartitionsJob implements Jobby
   {
     private final HadoopDruidIndexerConfig config;
     private final String partitionDimension;
+    private final Map<DateTime, Integer> intervalIndexes;
 
     public DeterminePartitionsDimSelectionMapperHelper(HadoopDruidIndexerConfig config, String partitionDimension)
     {
       this.config = config;
       this.partitionDimension = partitionDimension;
+
+      final ImmutableMap.Builder<DateTime, Integer> timeIndexBuilder = ImmutableMap.builder();
+      int idx = 0;
+      for(final Interval bucketInterval: config.getGranularitySpec().bucketIntervals()) {
+        timeIndexBuilder.put(bucketInterval.getStart(), idx);
+        idx ++;
+      }
+
+      this.intervalIndexes = timeIndexBuilder.build();
     }
 
     public void emitDimValueCounts(
@@ -391,7 +406,12 @@ public class DeterminePartitionsJob implements Jobby
       }
 
       final Interval interval = maybeInterval.get();
-      final byte[] groupKey = interval.getStart().toString().getBytes(Charsets.UTF_8);
+      final int intervalIndex = intervalIndexes.get(interval.getStart());
+
+      final ByteBuffer buf = ByteBuffer.allocate(4 + 8);
+      buf.putInt(intervalIndex);
+      buf.putLong(interval.getStartMillis());
+      final byte[] groupKey = buf.array();
 
       // Emit row-counter value.
       write(context, groupKey, new DimValueCount("", "", 1));
@@ -414,6 +434,24 @@ public class DeterminePartitionsJob implements Jobby
     }
   }
 
+  public static class DeterminePartitionsDimSelectionPartitioner
+    extends Partitioner<BytesWritable, Text>
+  {
+    @Override
+    public int getPartition(BytesWritable bytesWritable, Text text, int numPartitions)
+    {
+      final ByteBuffer bytes = ByteBuffer.wrap(bytesWritable.getBytes());
+      bytes.position(4); // Skip length added by SortableBytes
+      final int index = bytes.getInt();
+
+      if (index >= numPartitions) {
+        throw new ISE("Not enough partitions, index[%,d] >= numPartitions[%,d]", index, numPartitions);
+      }
+
+      return index;
+    }
+  }
+
   private static abstract class DeterminePartitionsDimSelectionBaseReducer
       extends Reducer<BytesWritable, Text, BytesWritable, Text>
   {
@@ -511,7 +549,9 @@ public class DeterminePartitionsJob implements Jobby
         Context context, SortableBytes keyBytes, Iterable<DimValueCount> combinedIterable
     ) throws IOException, InterruptedException
     {
-      final DateTime bucket = new DateTime(new String(keyBytes.getGroupKey(), Charsets.UTF_8));
+      final ByteBuffer groupKey = ByteBuffer.wrap(keyBytes.getGroupKey());
+      groupKey.position(4); // Skip partition
+      final DateTime bucket = new DateTime(groupKey.getLong());
       final PeekingIterator<DimValueCount> iterator = Iterators.peekingIterator(combinedIterable.iterator());
 
       log.info(
diff --git a/indexer/src/main/java/com/metamx/druid/indexer/IndexGeneratorJob.java b/indexer/src/main/java/com/metamx/druid/indexer/IndexGeneratorJob.java
index 874a30b7ad4..2c8cf82d822 100644
--- a/indexer/src/main/java/com/metamx/druid/indexer/IndexGeneratorJob.java
+++ b/indexer/src/main/java/com/metamx/druid/indexer/IndexGeneratorJob.java
@@ -220,7 +220,7 @@ public class IndexGeneratorJob implements Jobby
     public int getPartition(BytesWritable bytesWritable, Text text, int numPartitions)
     {
       final ByteBuffer bytes = ByteBuffer.wrap(bytesWritable.getBytes());
-      bytes.position(4); // Skip length added by BytesWritable
+      bytes.position(4); // Skip length added by SortableBytes
       int shardNum = bytes.getInt();
 
       if (shardNum >= numPartitions) {