private void evaluateStruct(ColumnVector colVector) {
    StructColumnVector scv = (StructColumnVector) colVector;
    scv.isRepeating = true;
    if (!isNullValue) {
      scv.isNull[0] = false;
      final int size = structValue.length;
      for (int i = 0; i < size; i++) {
        structValue[i].evaluateColumn(scv.fields[i]);
      }
    } else {
      scv.isNull[0] = true;
      scv.noNulls = false;
    }
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/ConstantVectorExpression.java