Comments - Adding columns to a table with full text column extremely slow

There are many limitations around FULLTEXT INDEX in InnoDB: https://mariadb.com/resources/blog/initial-impressions-of-innodb-fulltext/

MDEV-17459 prevents the ADD COLUMN from being instantaneous. To add insult to the injury, if an ALTER TABLE operation would involve creating multiple FULLTEXT INDEX (like it would if the table were to be rebuilt for adding the columns), then InnoDB would refuse a native operation, and the inefficient ALGORITHM=COPY would be used.

You could try dropping all fulltext indexes first, and then perform ALTER TABLE t FORCE, ADD COLUMN …, ADD FULLTEXT INDEX …, ALGORITHM=INPLACE (creating at most one fulltext index). The FORCE keyword prevents ALGORITHM=INSTANT operation and forces a rebuild. It should not be necessary, but I did not test it.

If there are multiple fulltext indexes, you can create them one at a time with subsequent ALTER TABLE t ADD FULLTEXT INDEX statements. Note: when you add the first fulltext index, a hidden FTS_DOC_ID column will be created inside InnoDB. So, typically adding the first fulltext index will require the table to be rebuilt. Also, note that when any FULLTEXT or SPATIAL index are to exist after the ALTER TABLE, the operation cannot be executed online (while allowing concurrent writes to the table).