@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", " + getColumnParamString(1, inputColumnNum[1]) +
        ", val "+ displayUtf8Bytes(arg3Scalar);
  }
Compute IF(expr1, expr2, expr3) for 3 input expressions.
The first is always a boolean (LongColumnVector).
The second is a string column expression.
The third is a string scalar.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/IfExprStringGroupColumnStringScalar.java