@Override
  public String vectorExpressionParameters() {
    return expr.getExprString();
  }
A VectorUDFAdaptor is a vectorized expression for invoking a custom
UDF on zero or more input vectors or constants which are the function arguments.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/udf/VectorUDFAdaptor.java