@Override
    public ByteSegmentRef first() {
      if (!hasRows) {
        return null;
      }

      // Position to beginning.
      readIndex = 0;

      return internalRead();
    }
Get detailed HashMap result position information to help diagnose exceptions.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/fast/VectorMapJoinFastValueStore.java