```ruby
class AddIndexToProjectAuthorizations < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  disable_ddl_transaction!

  def up
    add_concurrent_index(:project_authorizations, :project_id)
  end

  def down
    remove_index(:project_authorizations, :project_id)
  end
end
```

Drop the index only for postgresql, because
user
```ruby
class AddIndexToProjectAuthorizations < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  disable_ddl_transaction!

  def up
    add_concurrent_index(:project_authorizations, :project_id)
  end

  def down
    remove_index(:project_authorizations, :project_id) if
      Gitlab::Database.postgresql?
  end
end
```
assistant