@Override
  public String vectorExpressionParameters() {
    String value;
    if (isNullValue) {
      value = "null";
    } else {
      switch (type) {
      case LONG:
        value = Long.toString(longValue);
        break;
      case DOUBLE:
        value = Double.toString(doubleValue);
        break;
      case BYTES:
        value = new String(bytesValue, StandardCharsets.UTF_8);
        break;
      case DECIMAL:
        value = decimalValue.toString();
        break;
      case TIMESTAMP:
        value = org.apache.hadoop.hive.common.type.Timestamp.ofEpochMilli(
            timestampValue.getTime(), timestampValue.getNanos()).toString();
        break;
      case INTERVAL_DAY_TIME:
        value = intervalDayTimeValue.toString();
        break;
      case STRUCT:
        {
          StringBuilder sb = new StringBuilder();
          sb.append("STRUCT {");
          boolean isFirst = true;
          final int size = structValue.length;
          for (int i = 0; i < size; i++) {
            if (isFirst) {
              isFirst = false;
            } else {
              sb.append(", ");
            }
            sb.append(structValue[i].toString());
          }
          sb.append("}");
          value = sb.toString();
        }
        break;
      default:
        throw new RuntimeException("Unknown vector column type " + type);
      }
    }
    return "val " + value;
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java