ALTER TABLE ADD COLUMN takes a long time

Question

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         


        

Answer 1

Your ALTER TABLE statement implies mysql will have to re-write every single row of the table including the new column. Since you have more than 2 million rows, I would definitely expect it takes a significant amount of time, during which your server will likely be mostly IO-bound. You'd usually find it's more performant to do the following:

CREATE TABLE main_table_new LIKE main_table;
ALTER TABLE main_table_new ADD COLUMN location VARCHAR(256);
INSERT INTO main_table_new SELECT *, NULL FROM main_table;
RENAME TABLE main_table TO main_table_old, main_table_new TO main_table;
DROP TABLE main_table_old;

This way you add the column on the empty table, and basically write the data in that new table that you are sure no-one else will be looking at without locking as much resources.