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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def count(self):
 """ Returns the number of rows matched by this query. *Note: This function executes a SELECT COUNT() and has a performance cost on large datasets* """

        if self._batch:
            raise CQLEngineException("Only inserts, updates, and deletes are available in batch mode")

        if self._count is None:
            query = self._select_query()
            query.count = True
            result = self._execute(query)
            count_row = result.one().popitem()
            self._count = count_row[1]
        return self._count