private void evaluateLong(ColumnVector colVector) {

    LongColumnVector cv = (LongColumnVector) colVector;
    cv.isRepeating = true;
    if (!isNullValue) {
      cv.isNull[0] = false;
      cv.vector[0] = longValue;
    } else {
      cv.isNull[0] = true;
      cv.noNulls = false;
    }
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java