@Override
  public VectorExpressionDescriptor.Descriptor getDescriptor() {
    return (new VectorExpressionDescriptor.Builder())
        .setMode(
            VectorExpressionDescriptor.Mode.PROJECTION)
        .setNumArguments(2)
        .setArgumentTypes(
            VectorExpressionDescriptor.ArgumentType.STRING_FAMILY,
            VectorExpressionDescriptor.ArgumentType.STRING_FAMILY)
        .setInputExpressionTypes(
            VectorExpressionDescriptor.InputExpressionType.COLUMN,
            VectorExpressionDescriptor.InputExpressionType.COLUMN).build();
  }
Propagate nulls from input vector inV to output vector outV.

@param selectedInUse true/false flag to tell if sel[] is in use
@param sel selected value position array
@param n number of qualifying rows
@param inV input vector
@param outV output vector
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringGroupConcatColCol.java