I'm wondering if there's troubles (performance/load) to add a field
ALTER TABLE `lp_user` ADD `role` ENUM( 'user', 'querytool', 'admin' )
CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL DEFAULT 'user'
in a table with more than 500000 records ?
Thanks in advance.
Maybe. It depends on the load the server is under when you run the command, how much available memory there is, your configuration directives (myisam_sort_buffer_size for example), how many indexes are declared on the table, and if it does have to hit disk to create a temporary table in order to execute this ALTER statement, what kind of IO load your disk subsystem is under at the time.
These are all variables only you are aware of right now, so, maybe.