The best approach for creating a new column that has a default has changed since Postgres 11. In the case where we want the existing rows to have a different value from future created rows (e.g. a boolean which signifies if a user should see the onboarding tour), the best approach has also improved significantly in a way that perhaps isn't immediately intuitive.

In the before times, in the long-long-ago (before Postgres 11)

Before Postgres 11, we had to create new columns with defaults in a multi-step process to avoid locking the database.

-- not specifying default, because before pg 11 this would lock and rewrite the entire table
ALTER TABLE users
  ADD COLUMN has_seen_tour BOOLEAN;

ALTER TABLE users
  ALTER COLUMN has_seen_tour SET DEFAULT FALSE,
  ALTER COLUMN has_seen_tour SET NOT NULL;

-- to facilitate backfill below
CREATE INDEX CONCURRENTLY my_index ON users (has_seen_tour);

User.where(has_seen_tour: nil).in_batches do |batch|
  sleep 0.01
  batch.update_all(has_seen_tour: FALSE)
end

DROP INDEX CONCURRENTLY my_index;

>= Postgres 11

Now, this process is much more simple!

ALTER TABLE users
  ADD COLUMN has_seen_tour BOOLEAN NOT NULL DEFAULT FALSE;

That's it!

However...

Populating existing rows with a different value from the default

Let's say you want the existing rows to be initialized with a different value from future
new rows. Your first instinct might be to set the default for what you want new rows to have,
and then run a loop backfilling the old rows.

ALTER TABLE users
  ADD COLUMN has_seen_tour BOOLEAN NOT NULL DEFAULT FALSE;

Foo.in_batches do |batch|
  sleep 0.1
  batch.update_all(has_seen_tour: true)
end

However, thanks to Postgres 11's magic, there's a much easier way, without needing to do any table scanning or iterating through rows in a data migration:

-- initialize existing rows with FALSE
ALTER TABLE users
  ADD COLUMN has_seen_tour BOOLEAN NOT NULL DEFAULT TRUE;
​
-- all new rows going forward should start with FALSE
ALTER TABLE users
  ALTER COLUMN has_seen_tour SET DEFAULT FALSE;

That's it!