add generated virtual column still copy data

MySQL & MariaDB Other MySQL® Questions
1

When run the below query it take long time
alter table record_log add column record_key varchar(255) AS (md5(concat(record_time,record_campId))) VIRTUAL ;
show processlist show it copy data to tmp table, suppose it will done in place with data copy

34 | root | localhost | record | Query | 26 | copy to tmp table | alter table record_log add column record_key varchar(32) AS (md5(concat(record_time,reco | 0 |

MySQL version is 5.7.13-6-log Percona Server (GPL), Release 6, Revision e3d58bb

Thanks