private void initTopLevelField(int logicalColumnIndex, int projectionColumnNum,
      TypeInfo sourceTypeInfo) {

    projectionColumnNums[logicalColumnIndex] = projectionColumnNum;

    topLevelFields[logicalColumnIndex] = allocateField(sourceTypeInfo);
  }
@param deserializeRead   Set useExternalBuffer to true to avoid buffer copying and to get
    more efficient reading.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorDeserializeRow.java