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Returns the comparison result of the next row from each cursor. If one cursor has no more rows but the other does then subsequent calls to this will indicate that the remaining rows are unique.
The caller must check that hasNext() returns true before calling this.
Once next() has been called the cursors specified in the result of the call to next() are guaranteed to point to the row that was indicated. Reading values from the cursor that was not indicated in the call to next() will result in undefined behavior.