private void evaluateColumn(ColumnVector colVector) {
    switch (type) {
    case LONG:
      evaluateLong(colVector);
      break;
    case DOUBLE:
      evaluateDouble(colVector);
      break;
    case BYTES:
      evaluateBytes(colVector);
      break;
    case DECIMAL:
      if (outputDataTypePhysicalVariation == DataTypePhysicalVariation.DECIMAL_64) {
        evaluateDecimal64(colVector);
      } else {
        evaluateDecimal(colVector);
      }
      break;
    case TIMESTAMP:
      evaluateTimestamp(colVector);
      break;
    case INTERVAL_DAY_TIME:
      evaluateIntervalDayTime(colVector);
      break;
    case STRUCT:
      evaluateStruct(colVector);
      break;
    case VOID:
      evaluateVoid(colVector);
      break;
    default:
      throw new RuntimeException("Unexpected column vector type " + type);
    }
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java