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.

Answer 2

I think the appropriate answer for this is using a feature like pt-online-schema-change or gh-ost.

We have done migration of over 4 billion rows with this, though it can take upto 10 days, with less than a minute of downtime.

Percona works in a very similar fashion as above

• Create a temp table
• Creates triggers on the first table (for inserts, updates, deletes) so that they are replicated to the temp table
• In small batches, migrate data
• When done, rename table to new table, and drop the other table

Answer 3

Alter table takes a long time with a big data like in your case, so avoid to use it in such situations, and use some code like this one:

select main_table.*, 
  cast(null as varchar(256)) as null_location, -- any column you want accepts null
  cast('' as varchar(256)) as not_null_location, --any column doesn't accept null
  cast(0 as int) as not_null_int, -- int column doesn't accept null
into new_table 
from main_table;

drop table main_table;
rename table new_table TO main_table;