docs: function numberOfDistinctValues(node, context, columnName, callback) {
      var sql = countDistinctValuesTemplate({
          source: node.sql(),
          column: columnName
      });
      context.runSQL(sql, function(err, resultSet){
          var number = null;
          if (!err) {
              number = resultSet.rows[0].count_distict_values;
          }
          return callback(err, number);
      });
  }