@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]);
  }
Cast decimal(p1, s1) to decimal(p2, s2).

The precision and scale are recorded in the input and output vectors,
respectively.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/CastDecimalToDecimal.java