code: function numberOfRows(node, context, callback) {
      var sql = exactCountTemplate({
          sourceQuery: node.sql()
      });
      context.runSQL(sql, function(err, resultSet){
          var counted_rows = null;
          if (!err) {
              counted_rows = resultSet.rows[0].result_rows;
          }
          return callback(err, counted_rows);
      });
  }
docs: Number of rows in the result of the node. A LimitsContext object must be provided. This is an asynchronous method; an error object and the number of estimated rows will be passed to the callback provided. This can be slow for large tables or complex queries.