private void evaluateVoid(ColumnVector colVector) {
    VoidColumnVector voidColVector = (VoidColumnVector) colVector;
    voidColVector.isRepeating = true;
    voidColVector.isNull[0] = true;
    voidColVector.noNulls = false;
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java