@Override
  public void prepareToWrite(RecordWriter writer) throws IOException {
    this.writer = writer;
    computedSchema = (HCatSchema) ObjectSerializer.deserialize(UDFContext.getUDFContext().getUDFProperties(this.getClass(), new String[]{sign}).getProperty(COMPUTED_OUTPUT_SCHEMA));
  }
Here we are processing HCat table schema as derived from metastore, 
thus it should have information about all fields/sub-fields, but not for partition columns
hcatalog/hcatalog-pig-adapter/src/main/java/org/apache/hive/hcatalog/pig/HCatBaseStorer.java