[veddan]

I'm a bit behind on modern PostgreSQL so I might be wrong, but I believe MySQL has better support for what they call "online DDL" (modifying tables without blocking simultaneous queries).

Last time I checked, MySQL supported it in more cases. MySQL can also be explicit about it via "lock assertions":

  ALTER TABLE ..., LOCK=NONE;

will give an error if the requested operation can't be performed while still allowing concurrent reads and writes to the table (if you're fine with preventing writes you can use LOCK=SHARED).

The LOCK clause isn't just an assertion and can actually affect how MySQL performs the operation, but I tend to think of it as asserting "this won't cause downtime by locking this multi billion-row table while it spends an hour rewriting it".