Why “tokudb_disable_slow_alter=on” disallow slow alter statement which is original slow?

[郭枫]

I know “tokudb_disable_slow_alter=on” should disallow slow alter statement when a alter could be writen in two ways--one is fast alter,and the other is slow alter;But if an alter statement has just one form--slow alter form, I think this alter statement should be allowed~

For example: if Fmemo is varchar(64), I want to change it to varcahr(32),and tokudb doesn't support hot varcahr shrink,so it's a slow alter,but using  “tokudb_disable_slow_alter=on” just disable shrink alter statement:

MariaDB [c2c_db_50_2015]> alter table test modify Fmemo varchar(32);  
ERROR 1112 (42000): Table 'test' uses an extension that doesn't exist in this MariaDB version
MariaDB [c2c_db_50_2015]> alter table test change Fmemo Fmemo varchar(32);  
ERROR 1112 (42000): Table 'test' uses an extension that doesn't exist in this MariaDB version

[Rik Prohaska]

The tokudb_disable_slow_alter variable exists primarily for test purposes.