public VectorExpressionWriter init(SettableUnionObjectInspector objInspector) throws HiveException {
        super.init(objInspector);
        obj = initValue(null);
        vectorExtractRow = new VectorExtractRow();
        unionTypeInfo = (UnionTypeInfo)
            TypeInfoUtils.getTypeInfoFromTypeString(objInspector.getTypeName());
        return this;
      }
Compiles the appropriate vector expression writers based on a struct object
inspector.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorExpressionWriterFactory.java