When adding or removing a non-materialized virtual column to/from a MyISAM table with ALTER TABLE, MariaDB recreates the whole table although only the metadata should be affected:

MariaDB [test]> ALTER TABLE test ADD COLUMN testcolumn INT AS (othercolumn+1) VIRTUAL;

Stage: 1 of 2 'copy to tmp table'  0.001% of stage done

This is especially annoying for multi-gigabyte tables, where this operation takes a long time.