Default (empty) field list in partitioning by KEY() clause is assigned
from primary key. If primary key is changed the partitioning field
list is changed as well, so repartitioning required. Not applicable to
any non-primary keys as default field list may be taken only from
primary key.
MyISAM only allows online alter if autoincrement didn't change.
MyISAM detects that by comparing new autoinc value from create_info,
with the old one, stored in MYI. But in partitioned tables,
create_info->auto_increment_value is for the whole table, max of
autoinc values of individual MYI partitions. So *some* MYI partitions
will inevitably think that alter table changes auto_increment value
and will deny online alter.
Fix: only compare autoinc values, if the user has used AUTO_INCREMENT
in the ALTER TABLE statement.
