MySQL Schema – How to Dynamically Change Schema to Accommodate Custom Fields

My first quesiton is, what is "very huge?" Depending on the answer to that question, there may be other ways to optimize your migrations to reduce the time that they take.

Second question is, how often are you making schema changes on your live application?

If the online schema change tools don't work for you, you could try this from another angle. Use master/master replication in active/passive mode to minimize your downtime.

The following assumes that you have db1 and db2 in an active/passive master/master configuration...

1. Perform the schema change on the db2 with binary logging disabled (SET SQL_LOG_BIN=0 in the session before you perform the ALTER TABLE)
2. Point your application at db2, thus making it the active server
3. Perform schema change on db1 with binary logging disabled
4. optionally, switch back to db1

For this to work, your schema migrations have to be backwards compatible, which is likely the case anyhow in most applications. This isn't zero downtime, but it is close. Realistically, most applications can tolerate a small amount of downtime periodically for maintenance. True zero downtime is a very expensive luxury.

To potentially speed up your table alters, look into using Percona Server and the expand_fast_index_creation setting.

Also, another option for online schema changes is Facebook's OSC.

Is there a reason why you have no unique constraints in your database? What happens if somehow you get two rows with configuration data for the same email address? How do you determine which one to use?

I think the one piece of real advice I can give you is look at keys/functional dependencies and add as many unique constraints as make sense. it is far easier to drop unique constraints later than it is to add them.