@Override
  public Category getCategory() {
    return Category.STRUCT;
  }
Create a new Object Inspector based on a base object inspector and the subset of
columns will be inspected (from startCol to startCol+numCols).
@param baseOI
@param startCol
@param numCols
serde/src/java/org/apache/hadoop/hive/serde2/objectinspector/SubStructObjectInspector.java