protected static void validateComparable(ObjectInspector OI, String errMsg)
      throws SemanticException {
    if (!ObjectInspectorUtils.compareSupported(OI)) {
      throw new SemanticException(errMsg);
    }
  }
For each column on the input RR, construct a StructField for it
OI is constructed using the list of input column names and
their corresponding OIs.

@param rr
@return
ql/src/java/org/apache/hadoop/hive/ql/parse/PTFTranslator.java