use `add_concurrent_index` to add the :confirmation_token index

db/migrate/20170904092148_add_email_confirmation.rb

@@ -29,6 +29,5 @@ class AddEmailConfirmation < ActiveRecord::Migration
     add_column :emails, :confirmation_token, :string
     add_column :emails, :confirmed_at, :datetime
     add_column :emails, :confirmation_sent_at, :datetime
-    add_index  :emails, :confirmation_token, unique: true
   end
 end

db/migrate/20170909090114_add_email_confirmation_index.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddEmailConfirmationIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  # DOWNTIME_REASON = ''
+
+  # When using the methods "add_concurrent_index", "remove_concurrent_index" or
+  # "add_column_with_default" you must disable the use of transactions
+  # as these methods can not run in an existing transaction.
+  # When using "add_concurrent_index" or "remove_concurrent_index" methods make sure
+  # that either of them is the _only_ method called in the migration,
+  # any other changes should go in a separate migration.
+  # This ensures that upon failure _only_ the index creation or removing fails
+  # and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  disable_ddl_transaction!
+
+  # Not necessary to remove duplicates, as :confirmation_token is a new column
+  def up
+    add_concurrent_index :emails, :confirmation_token, unique: true
+  end
+
+  def down
+    remove_index :emails, :confirmation_token if index_exists?(:emails, :confirmation_token)
+  end
+end