private void evaluateTimestamp(ColumnVector colVector) {
    TimestampColumnVector tcv = (TimestampColumnVector) colVector;
    tcv.isRepeating = true;
    if (!isNullValue) {
      tcv.isNull[0] = false;
      tcv.set(0, timestampValue);
    } else {
      tcv.isNull[0] = true;
      tcv.noNulls = false;
    }
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java