Depends on what scale you're dealing with. If you have a high traffic site, the db should be sharded so if you do a manual switch master-slave, only a small pi

Depends on what scale you're dealing with. If you have a high traffic site, the db should be sharded so if you do a manual switch master-slave, only a small piece would be affected at a given time. But I'm guessing you're dealing with a single M-S setup. I've asked around and it seems the standard practice for that type of a setup is to create a second table for each one you are attempting to modify, 'insert into table2 select * from table 1;', modify table2, rename table 1, rename table 2 to table 1. Then, script or manually cope with any 'leftovers' in table 1 that didn't get ported to table 2. Would be interesting to have an in-memory (but written to disk) NoSQL layer sandwiched between MySQL and the user. Then, you can change schema all you want or switch in/out DB servers without any visible impact. Might be a leaky abstraction though. Not like I tried that out.