@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", " + getColumnParamString(1, inputColumnNum[1]) +
        getColumnParamString(2, inputColumnNum[2]);
  }
Compute IF(expr1, expr2, expr3) for 3 input column expressions.
The first is always a boolean (LongColumnVector).
The second and third are long columns or long expression results.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/IfExprDoubleColumnDoubleColumn.java