This is a command-line option for the
Sorts each table's rows by primary key, or first unique key, if such a key exists. Useful when dumping a MyISAM table to be loaded into an InnoDB table, but will make the dump itself take considerably longer.
This page shows information specific to MariaDB SkySQL. Information for non-SkySQL use can be found on the order-by-primary page in the MariaDB Enterprise Documentation.