How to change column type (e.g., int -> bigint) without downtime

change_column_type.rb

ActiveRecord::Migration.remove_foreign_key(:current_table, :foreign_table) # no lock

ActiveRecord::Migration.add_column(:current_table, :column_bigint, :bigint) # no lock

copy_data = lambda do
  CurrentTable.where(column_bigint: nil).where.not(column: nil).in_batches do |batch|
    batch.update_all("column_bigint = column")
  end
end

copy_data.call

ActiveRecord::Migration.remove_index :current_table, :column, algorithm: :concurrently # no lock

CurrentTable.transaction do
  ActiveRecord::Migration.execute "LOCK TABLE current_table IN ACCESS EXCLUSIVE MODE"
  copy_data.call # should be fast
  ActiveRecord::Migration.rename_column :current_table, :column, :column_int # no lock
  ActiveRecord::Migration.rename_column :current_table, :column_bigint, :column # no lock
end

ActiveRecord::Migration.add_index :current_table, :column, algorithm: :concurrently # no lock

CurrentTable.transaction do
  ActiveRecord::Migration.add_foreign_key :current_table, :foreign_table, validate: false # no lock
end

CurrentTable.transaction do
  ActiveRecord::Migration.validate_foreign_key :current_table, :foreign_table # row level lock
end

ActiveRecord::Migration.remove_column(:current_table, :column_int)