Question

We have a very large (10million+) row table stored in MySql using the InnoDB engine. Column ‘x’ is defined as ‘smallint(5) unsigned not null’.

Requirements have now changed since the original design a few years ago, and column ‘x’ needs to store a minimum datatype size of ‘int unsigned not null’.

We are allowed a “short” downtime to deploy the application (less than 5 mins) so any database change would need to fit within this time window if it requires the database table to be made temporarily unavailable in any way (e.g. full table lock). If the change can be done “online” then we can probably accept degraded performance for a longer period.

Has anyone here had experience of altering column type in MySql/InnoDB on a very large table? If so, did it go OK and roughly how long did it take (I realise this is hardware dependent, I am just trying to understand if what we are asking to do in the time window is even vaguely possible)?

Thanks in advance,

Answer 1

heres a recipe i’ve used

where you have old column, a

1. create a new column b
2. create a trigger to update b on update/insert on a
3. update b = a
4. drop all fkey relations referencing a
5. create fkey relations for b
6. update code to use column b instead of a (deploy)
7. drop triggers on column a
8. drop column a

repeat all steps if you must change the column name back.