@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", " + getColumnParamString(1, inputColumnNum[1]);
  }
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