This topic is linked to MDL-48228.
We are trying to convert our MySQL DB to utf8mb4_unicodeCI.
The problem is : using admin/cli/mysql_collation.php for the conversion doesn't work with big databases. So I tried what was suggested in the comment : I created a dump file and changed its collations with a sed command.
But the result is different from a database changed with mysql_collation.php. mysql_collation.php only sets the compressed format on the tables that need it. With a sed, all the tables are changed.
I also tried to create a new instance of Moodle with a utf8mb4 DB (to fill it with a dump containing only data). I launched admin/cli/install_database.php, but then all the tables had the format compressed!
Do you think it's a problem that all the tables are compressed?