How to use the sharedb.DB.call function in sharedb

function MySQLDB(options) {
	if (!(this instanceof MySQLDB)) return new MySQLDB(options);
	DB.call(this, options);

	this.closed = false;

	mysql_config = options.db;

	// connections in pool
	mysql_config.connectionLimit = options.db.connectionLimit ? options.connectionLimit : 10;
	  
	pool  = mysql.createPool(mysql_config);

	if (options.debug) {
		debug = options.debug;
		pool.on('acquire', function (connection) {
			console.log('Connection %d acquired', connection.threadId);
		});

function PostgresDB(options) {
  if (!(this instanceof PostgresDB)) return new PostgresDB(options);
  DB.call(this, options);

  this.closed = false;

  this.pg_config = options;
};
module.exports = PostgresDB;