How to Create Postgres Indexes Concurrently in ActiveRecord Migrations

之前的文章提到要避免 deployment downtime 其中一個方式是 create index concurrently
http://everyday1percent.blogspot.com/2018/05/rails-deployment-downtime.html

原來 rails 有內建的 API 做到這件事

```ruby
class AddIndexToBuildMetricIdOnBuilds < ActiveRecord::Migration
  disable_ddl_transaction!

  def change
    add_index :builds, :build_metric_id, :algorithm => :concurrently
  end
end
```

好潮。




https://robots.thoughtbot.com/how-to-create-postgres-indexes-concurrently-in
https://semaphoreci.com/blog/2017/06/21/faster-rails-indexing-large-database-tables.html