Sort operator
- this->rows = new vector<Row*>(): Allocate storage
to accumulate all the input rows.
- Sort::open:
- while (row = input->next() { rows->emplace_back(row); }: Read all of the input rows.
- std::sort(...);: Sort the rows.
- this->output = rows->begin(): Create a (C++) iterator
to visit the sorted rows.
- In Sort::next(), return the next sorted row: next = output++.