ALTER TABLE without locking the table?

Question

When doing an ALTER TABLE statement in MySQL, the whole table is read-locked (allowing concurrent reads, but prohibiting concurrent writes) for the duration of the statement

Answer 1

I recommend Postgres if that's an option. With postgres there is essentially no downtime with the following procedures:

• ALTER TABLE ADD COLUMN (if the column can be NULL)
• ALTER TABLE DROP COLUMN
• CREATE INDEX (must use CREATE INDEX CONCURRENTLY)
• DROP INDEX

Other great feature is that most DDL statements are transactional, so you could do an entire migration within a SQL transaction, and if something goes wrong, the entire thing gets rolled back.

I wrote this a little bit ago, perhaps it can shed some more insight on the other merits.