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Iterators


class Iterator
{
public:
    void open() = 0;
    Row* next() = 0;
    void close() = 0;
};

How Iterators will be used

A query execution plan is built out of operators.
These operators are like Relational Algebra operators, but they also specify an implementation, e.g.
- SelectScan: Implement the select operator by scanning the entire input and applying a predicate to each row.
- SelectIndex: Implement the select operator by using an index.
Each of these operators is an Iterator.

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