alter mysql performance

ALTER TABLE ADD COLUMN takes a long time

I was just trying to add a column called “location” to a table (main_table) in a database. The command I run was

ALTER TABLE main_table ADD COLUMN location varchar (256);

The main_table contains > 2,000,000 rows. It keeps running for more than 2 hours and still not completed.

I tried to use mytop
to monitor the activity of this database to make sure that the query is not locked by other querying process, but it seems not. Is it supposed to take that long time? Actually, I just rebooted the machine before running this command. Now this command is still running. I am not sure what to do.