Just for reference, as of PG11 it no longer locks or rewrites for default column creation.
It's one of the nice things about Postgres, they're always improving. A few years ago, concurrent index creation wasn't a thing either. Nor were "CREATE/DROP IF (NOT) EXIST" statements for various bits and pieces, but they just keep adding to it over time.
In the upcoming PG12 there is REINDEX CONCURRENTLY, so we'll finally be able to fix corrupt/bloated indexes on the fly without having to drop and recreate them.
It's one of the nice things about Postgres, they're always improving. A few years ago, concurrent index creation wasn't a thing either. Nor were "CREATE/DROP IF (NOT) EXIST" statements for various bits and pieces, but they just keep adding to it over time.
In the upcoming PG12 there is REINDEX CONCURRENTLY, so we'll finally be able to fix corrupt/bloated indexes on the fly without having to drop and recreate them.